George Brinton McClellan (December 3, 1826 – October 29, 1885) was a major general during the American Civil War. He organized the famous Army of the Potomac and served briefly (November 1861 to March 1862) as the general-in-chief of the Union Army. Early in the war, McClellan played an important role in raising a well trained and organized army for the Union. Although McClellan was meticulous in his planning and preparations, these characteristics may have hampered his ability to challenge aggressive opponents in a fast moving battlefield environment. He chronically overestimated the strength of enemy units and was reluctant to apply principles of mass, frequently leaving large portions of his army unengaged at decisive points.

McClellan's Peninsula Campaign in 1862 ended in failure, with retreats from attacks by General Robert E. Lee's smaller Army of Virginia and an unfulfilled plan to seize the Confederate capital of Richmond. His performance at the bloody Battle of Antietam blunted Lee's invasion of Maryland, but allowed Lee to eke out a precarious tactical draw and avoid destruction, despite being outnumbered. As a result, McClellan's leadership skills during battles were questioned by U.S. President Abraham Lincoln, who eventually removed him from command, first as general-in-chief, then from the Army of the Potomac. Lincoln offered this famous evaluation of McClellan: "If he can't fight himself, he excels in making others ready to fight." Indeed, McClellan was the most popular of that army's commanders with its soldiers, who felt that he had their morale and well-being as paramount concerns. General McClellan also failed to maintain the trust of Lincoln, and proved to be frustratingly derisive of, and insubordinate to, his commander-in-chief. After he was relieved of command, McClellan became the unsuccessful Democratic nominee opposing Lincoln in the 1864 presidential election. His party had an anti-war platform, promising to end the war and negotiate with the Confederacy, which McClellan was forced to repudiate, damaging the effectiveness of his campaign. He served as the 24th Governor of New Jersey from 1878 to 1881. He eventually became a writer, defending his actions during the Peninsula Campaign and the Civil War. Although the majority of modern authorities assess McClellan as a poor battlefield general, a small but vocal faction of historians maintain that he was a highly capable commander, but his reputation suffered unfairly at the hands of pro-Lincoln partisans who needed a scapegoat for the Union's setbacks. His legacy therefore defies easy categorization. After the war, Ulysses S. Grant was asked to evaluate McClellan as a general. He replied, "McClellan is to me one of the mysteries of the war."

McClellan was born in Philadelphia, the son of a prominent surgical ophthalmologist, Dr. George McClellan, the founder of Jefferson Medical College. His mother was Elizabeth Steinmetz Brinton McClellan, daughter of a leading Pennsylvania family, a woman noted for her "considerable grace and refinement". The couple produced five children: a daughter, Frederica; then three sons, John, George, and Arthur; and a second daughter, Mary. McClellan was the grandson of Revolutionary War general Samuel McClellan of Woodstock, Connecticut. He first attended the University of Pennsylvania in 1840 at age 13, resigning himself to the study of law. After two years, he changed his goal to military service. With the assistance of his father's letter to President John Tyler, young George was accepted at the United States Military Academy in 1842, the academy having waived its normal minimum age of 16.

At West Point, he was an energetic and ambitious cadet, deeply interested in the teachings of Dennis Hart Mahan and the theoretical strategic principles of Antoine-Henri Jomini. His closest friends were aristocratic Southerners such as James Stuart, Dabney Maury, Cadmus Wilcox, and A.P. Hill. These associations gave McClellan what he considered to be an appreciation of the Southern mind, an understanding of the political and military implications of the sectional differences in the United States that led to the Civil War. He graduated in 1846, second in his class of 59 cadets, losing the top position (to Charles Seaforth Stewart) only because of poor drawing skills. He was commissioned a brevet second lieutenant in the U.S. Army Corps of Engineers.

McClellan's first assignment was with a company of engineers formed at West Point, but he quickly received orders to sail for the Mexican - American War. He arrived near the mouth of the Rio Grande in October 1846, well prepared for action with a double barreled shotgun, two pistols, a saber, a dress sword, and a Bowie knife. He complained that he had arrived too late to take any part in the American victory at Monterrey in September. During a temporary armistice in which the forces of Gen. Zachary Taylor awaited action, McClellan was stricken with dysentery and malaria, which kept him in the hospital for nearly a month. The malaria would recur in later years — he called it his "Mexican disease." He served bravely as an engineering officer during the war, subjected to frequent enemy fire, and was appointed a brevet first lieutenant for Contreras and Churubusco and to captain for Chapultepec. He performed reconnaissance missions for Lt. Gen. Winfield Scott, a close friend of McClellan's father.

McClellan's experiences during the war developed various attitudes that affected his later military and political life. He learned to appreciate the value of flanking movements over frontal assaults (used by Scott at Cerro Gordo) and the value of siege operations (Vera Cruz). He witnessed Scott's success in balancing political with military affairs, and his good relations with the civil population as he invaded, enforcing strict discipline on his soldiers to minimize damage to their property. And he developed a disdain for volunteer soldiers and officers, particularly politicians who cared nothing for discipline and training.

McClellan returned to West Point to command his engineering company, which was attached to the academy for the purpose of training cadets in engineering activities. He chafed at the boredom of peacetime garrison service, although he greatly enjoyed the social life. In June 1851 he was ordered to Fort Delaware, a masonry work under construction on an island in the Delaware River, 40 miles (64 km) downriver from Philadelphia. In March 1852 he was ordered to report to Capt. Randolph B. Marcy at Fort Smith, Arkansas, to serve as second in command on an expedition to discover the sources of the Red River. By June the expedition reached the source of the north fork of the river and Marcy named a small tributary McClellan's Creek. Upon their return to civilization on July 28, they were astonished to find that they had been given up for dead. A sensational story had reached the press, which McClellan blamed on "a set of scoundrels, who seek to keep up agitation on the frontier in order to get employment from the Govt. in one way or other," that the expedition had been ambushed by 2,000 Comanches and killed to the last man.

In the fall of 1852, McClellan published a manual on bayonet tactics that he had translated from the original French. He also received an assignment to the Department of Texas, with orders to perform a survey of Texas rivers and harbors. In 1853 he participated in the Pacific Railroad surveys, ordered by Secretary of War Jefferson Davis, to select an appropriate route for the upcoming transcontinental railroad. McClellan surveyed the northern corridor along the 47th and 49th parallels from St. Paul to the Puget Sound. During this assignment, he demonstrated a tendency for insubordination toward senior political figures. Isaac Stevens, governor of the Washington Territory, became dissatisfied with McClellan's performance in scouting passes across the Cascade Range. (McClellan selected Yakima Pass without a thorough reconnaissance and refused the governor's order to lead a party through it in winter conditions, relying on faulty intelligence about the depth of snowpack in that area. He also neglected to find three greatly superior passes in the near vicinity, which would be the ones eventually used for railroads and interstate highways.) The governor ordered McClellan to turn over his expedition logbooks, but McClellan steadfastly refused, most likely because of embarrassing personal comments that he had made throughout.

Returning to the East, McClellan began courting Ellen Mary Marcy (1836 – 1915), the daughter of his former commander. Ellen, or Nelly, refused McClellan's first proposal of marriage, one of nine that she received from a variety of suitors, including his West Point friend, A.P. Hill. Ellen accepted Hill's proposal in 1856, but her family did not approve and he withdrew.

In June 1854, McClellan was sent on a secret reconnaissance mission to Santo Domingo at the behest of Jefferson Davis. McClellan assessed local defensive capabilities for the secretary. (The information was not used until 1870, when President Ulysses S. Grant unsuccessfully attempted to annex the Dominican Republic.) Davis was beginning to treat McClellan almost as a protégé, and his next assignment was to assess the logistical readiness of various railroads in the United States, once again with an eye toward planning for the transcontinental railroad. In March 1855, McClellan was promoted to captain and assigned to the 1st U.S. Cavalry regiment.

Because of his political connections and his mastery of French, McClellan received the assignment to be an official observer of the European armies in the Crimean War in 1855. Traveling widely, and interacting with the highest military commands and royal families, McClellan observed the siege of Sevastopol. Upon his return to the United States in 1856 he requested assignment in Philadelphia to prepare his report, which contained a critical analysis of the siege and a lengthy description of the organization of the European armies. He also wrote a manual on cavalry tactics that was based on Russian cavalry regulations. A notable failure of the observers, including McClellan, was that they neglected to explain the importance of the emergence of rifled muskets in the Crimean War, and how that would require fundamental changes in tactics for the coming Civil War. The Army adopted McClellan's cavalry manual and also his design for a saddle, the "McClellan Saddle", which he claimed to have seen used by Hussars in Prussia and Hungary. It became standard issue for as long as the U.S. horse cavalry existed and is currently used for ceremonies.

McClellan resigned his commission January 16, 1857, and, capitalizing on his experience with railroad assessment, became chief engineer and vice president of the Illinois Central Railroad and also president of the Ohio and Mississippi Railroad in 1860. He performed well in both jobs, expanding the Illinois Central toward New Orleans and helping the Ohio and Mississippi recover from the Panic of 1857. But despite his successes and lucrative salary ($10,000 per year), he was frustrated with civilian employment and continued to study classical military strategy assiduously. During the Utah War against the Mormons, he considered rejoining the Army. He also considered service as a filibuster in support of Benito Juárez in Mexico.

Before the outbreak of Civil War, McClellan became active in politics, supporting the presidential campaign of Democrat Stephen A. Douglas in the 1860 election. He claimed to have defeated an attempt at vote fraud by Republicans by ordering the delay of a train that was carrying men to vote illegally in another county, enabling Douglas to win the county.

In October 1859 McClellan was able to resume his courtship of Ellen Marcy, and they were married in Calvary Church, New York City, on May 22, 1860.

At the start of the Civil War, McClellan's knowledge of what was called "big war science" and his railroad experience implied he would excel at military logistics. This placed him in great demand as the Union mobilized. The governors of Ohio, Pennsylvania, and New York, the three largest states of the Union, actively pursued him to command their states' militia. Ohio Governor William Dennison was the most persistent, so McClellan was commissioned a major general of volunteers and took command of the Ohio militia on April 23, 1861. Unlike some of his fellow Union officers who came from abolitionist families, he was opposed to federal interference with slavery. So some of his Southern colleagues approached him informally about siding with the Confederacy, but he could not accept the concept of secession.

On May 3 McClellan re-entered federal service by being named commander of the Department of the Ohio, responsible for the states of Ohio, Indiana, Illinois, and, later, western Pennsylvania, western Virginia, and Missouri. On May 14, he was commissioned a major general in the regular army. At age 34 he now outranked everyone in the Army other than Lt. Gen. Winfield Scott, the general in chief. McClellan's rapid promotion was partly because of his acquaintance with Salmon P. Chase, Treasury Secretary and former Ohio governor and senator.

As McClellan scrambled to process the thousands of men who were volunteering for service and to set up training camps, he also set his mind toward grand strategy. He wrote a letter to Gen. Scott on April 27, four days after assuming command in Ohio, that was the first proposal for a unified strategy for the war. It contained two alternatives, both with a prominent role for himself as commander. The first called for 80,000 men to invade Virginia through the Kanawha Valley toward Richmond. The second called for those same men to drive south instead across the Ohio River into Kentucky and Tennessee. Scott dismissed both plans as being logistically infeasible. Although he complimented McClellan and expressed his "great confidence in your intelligence, zeal, science, and energy", he replied by letter that the 80,000 men would be better used on a river-based expedition to control the Mississippi River and split the Confederacy, accompanied by a strong Union blockade of Southern ports. This plan, which would have demanded considerable patience on the part of the Northern public, was derided in newspapers as the Anaconda Plan, but eventually proved to be the successful outline used to prosecute the war. Relations between the two generals became increasingly strained over the summer and fall.

McClellan's first military operations were to occupy the area of western Virginia that wanted to remain in the Union and later became the state of West Virginia. He had received intelligence reports on May 26 that the critical Baltimore and Ohio Railroad bridges in that portion of the state were being burned. As he quickly implemented plans to invade the region, he triggered his first serious political controversy by proclaiming to the citizens there that his forces had no intentions of interfering with personal property — including slaves. "Notwithstanding all that has been said by the traitors to induce you to believe that our advent among you will be signalized by interference with your slaves, understand one thing clearly — not only will we abstain from all such interference but we will on the contrary with an iron hand, crush any attempted insurrection on their part." He quickly realized that he had overstepped his bounds and apologized by letter to President Lincoln. The controversy was not that his proclamation was diametrically opposed to the administration's policy at the time, but that he was so bold in stepping beyond his strictly military role.

His forces moved rapidly into the area through Grafton and were victorious at the tiny skirmish called the Battle of Philippi Races, arguably the first land conflict of the war. His first personal command in battle was at Rich Mountain, which he also won, but only after displaying a strong sense of caution and a reluctance to commit reserve forces that would be his hallmark for the rest of his career. His subordinate commander, William S. Rosecrans, bitterly complained that his attack was not reinforced as McClellan had agreed. Nevertheless, these two minor victories propelled McClellan to the status of national hero. The New York Herald entitled an article about him, "Gen. McClellan, the Napoleon of the Present War."

After the defeat of the Union forces at Bull Run on July 21, 1861, Lincoln summoned McClellan from West Virginia, where McClellan had given the North the only actions thus far having a semblance of military victories. He traveled by special train on the main Pennsylvania line from Wheeling through Pittsburgh, Philadelphia, and Baltimore, and on to Washington, D.C., and was overwhelmed by enthusiastic crowds that met his train along the way. Carl Sandburg wrote, "McClellan was the man of the hour, pointed to by events, and chosen by an overwhelming weight of public and private opinion." On July 26, the day he reached the capital, McClellan was appointed commander of the Military Division of the Potomac, the main Union force responsible for the defense of Washington. On August 20, several military units in Virginia were consolidated into his department and he immediately formed the Army of the Potomac, with himself as its first commander. He reveled in his newly acquired power and fame:

I find myself in a new and strange position here — Presdt, Cabinet, Genl Scott & all deferring to me — by some strange operation of magic I seem to have become the power of the land. ... I almost think that were I to win some small success now I could become Dictator or anything else that might please me — but nothing of that kind would please me — therefore I won't be Dictator. Admirable self-denial!

– George B. McClellan, letter to Ellen, July 26, 1861

McClellan's antipathy to emancipation added to the pressure on him, as he received bitter criticism from Radical Republicans in the government. He viewed slavery as an institution recognized in the Constitution, and entitled to federal protection wherever it existed (Lincoln held the same public position until August 1862). McClellan's writings after the war were typical of many Northerners: "I confess to a prejudice in favor of my own race, & can't learn to like the odor of either Billy goats or niggers." But in November 1861, he wrote to his wife, "I will, if successful, throw my sword onto the scale to force an improvement in the condition of those poor blacks." He later wrote that had it been his place to arrange the terms of peace, he would have insisted on gradual emancipation, guarding the rights of both slaves and masters, as part of any settlement. But he made no secret of his opposition to the radical Republicans. He told Ellen, "I will not fight for the abolitionists." This placed him at an obvious handicap because many politicians running the government believed that he was attempting to implement the policies of the opposition party.

The immediate problem with McClellan's war strategy was that he was convinced the Confederates were ready to attack him with overwhelming numbers. On August 8, believing that the Confederates had over 100,000 troops facing him (in contrast to the 35,000 they actually deployed at Bull Run a few weeks earlier), he declared a state of emergency in the capital. By August 19, he estimated 150,000 enemy to his front. McClellan's future campaigns would be strongly influenced by the overblown enemy strength estimates of his secret service chief, detective Allan Pinkerton, but in August 1861, these estimates were entirely McClellan's own. The result was a level of extreme caution that sapped the initiative of McClellan's army and caused great condemnation by his government. Historian and biographer Stephen W. Sears has called McClellan's actions "essentially sound" if he had been as outnumbered as he believed, but McClellan in fact rarely had less than a two-to-one advantage over his opponents in 1861 and 1862. That fall, for example, Confederate forces ranged from 35,000 to 60,000, whereas the Army of the Potomac in September numbered 122,000 men; in early December 170,000; by year end, 192,000.

The dispute with Scott would become very personal. Scott (along with many in the War Department) was outraged that McClellan refused to divulge any details about his strategic planning, or even mundane details such as troop strengths and dispositions. (For his part, McClellan claimed not to trust anyone in the administration to keep his plans secret from the press, and thus the enemy.) During disagreements about defensive forces on the Potomac River, McClellan wrote to his wife on August 10 in a manner that would characterize some of his more private correspondence: "Genl Scott is the great obstacle — he will not comprehend the danger & is either a traitor, or an incompetent. I have to fight my way against him." Scott became so disillusioned over his relationship with the young general that he offered his resignation to President Lincoln, who initially refused to accept it. Rumors traveled through the capital that McClellan might resign, or instigate a military coup, if Scott were not removed. Lincoln's Cabinet met on October 18 and agreed to accept Scott's resignation for "reasons of health." On November 1, 1861, Winfield Scott retired and McClellan became general in chief of all the Union armies. The president expressed his concern about the "vast labor" involved in the dual role of army commander and general in chief, but McClellan responded, "I can do it all."

Lincoln, as well as many other leaders and citizens of the northern states, became increasingly impatient with McClellan's slowness to attack the Confederate forces still massed near Washington. The Union defeat at the minor Battle of Ball's Bluff near Leesburg in October added to the frustration and indirectly damaged McClellan. In December, the Congress formed a Joint Committee on the Conduct of the War, which became a thorn in the side of many generals throughout the war, accusing them of incompetence and, in some cases, treason. McClellan was called as the first witness on December 23, but he contracted typhoid fever and could not attend. Instead, his subordinate officers testified, and their candid admissions that they had no knowledge of specific strategies for advancing against the Confederates raised many calls for McClellan's dismissal. McClellan further damaged his reputation by his insulting insubordination to his commander-in-chief. He privately referred to Lincoln, whom he had known before the war as a lawyer for the Illinois Central, as "nothing more than a well-meaning baboon", a "gorilla", and "ever unworthy of ... his high position." On November 13, he snubbed the president, visiting at McClellan's house, by making him wait for 30 minutes, only to be told that the general had gone to bed and could not see him.

On January 10, Lincoln met with top generals (McClellan did not attend) and directed them to formulate a plan of attack, expressing his exasperation with General McClellan with the following remark: "If General McClellan does not want to use the army, I would like to borrow it for a time." On January 12, 1862, McClellan was summoned to the White House, where the Cabinet demanded to hear his war plans. For the first time, he revealed his intentions to transport the Army of the Potomac by ship to Urbanna, Virginia, on the Rappahannock River, outflanking the Confederate forces near Washington, and proceeding 50 miles (80 km) overland to capture Richmond. He refused to give any specific details of the proposed campaign, even to his friend, newly appointed War Secretary Edwin M. Stanton. On January 27, Lincoln issued an order that required all of his armies to begin offensive operations by February 22, Washington's birthday. On January 31, he issued a supplementary order for the Army of the Potomac to move overland to attack the Confederates at Manassas Junction and Centreville. McClellan immediately replied with a 22-page letter objecting in detail to the president's plan and advocating instead his Urbanna plan, which was the first written instance of the plan's details being presented to the president. Although Lincoln believed his plan was superior, he was relieved that McClellan finally agreed to begin moving, and reluctantly approved. On March 8, doubting McClellan's resolve, Lincoln again interfered with the army commander's prerogatives. He called a council of war at the White House in which McClellan's subordinates were asked about their confidence in the Urbanna plan. They expressed their confidence to varying degrees. After the meeting, Lincoln issued another order, naming specific officers as corps commanders to report to McClellan (who had been reluctant to do so prior to assessing his division commanders' effectiveness in combat, even though this would have meant his direct supervision of twelve divisions in the field).

Two more crises would hit McClellan before he could implement his plans. The Confederate forces under General Joseph E. Johnston withdrew from their positions before Washington, assuming new positions south of the Rappahannock, which completely nullified the Urbanna strategy. McClellan retooled his plan so that his troops would disembark at Fort Monroe, Virginia, and advance up the Virginia Peninsula to Richmond, an operation that would be known as the Peninsula Campaign. However, McClellan came under extreme criticism from the press and the Congress when it was found that Johnston's forces had not only slipped away unnoticed, but had for months fooled the Union Army through the use of logs painted black to appear as cannons, nicknamed Quaker Guns. The Congress's joint committee visited the abandoned Confederate lines and radical Republicans introduced a resolution demanding the dismissal of McClellan, but it was narrowly defeated by a parliamentary maneuver. The second crisis was the emergence of the Confederate ironclad CSS Virginia, which threw Washington into a panic and made naval support operations on the James River seem problematic.

On March 11, 1862, Lincoln removed McClellan as general-in-chief, leaving him in command of only the Army of the Potomac, ostensibly so that McClellan would be free to devote all his attention to the move on Richmond. Lincoln's order was ambiguous as to whether McClellan might be restored following a successful campaign. In fact, his position was not filled by another officer. Lincoln, Stanton, and a group of officers called the "War Board" directed the strategic actions of the Union armies that spring. Although McClellan was assuaged by supportive comments Lincoln made to him, in time he saw the change of command very differently, describing it as a part of an intrigue "to secure the failure of the approaching campaign."

McClellan's army began to sail from Alexandria on March 17. It was an armada that dwarfed all previous American expeditions, transporting 121,500 men, 44 artillery batteries, 1,150 wagons, over 15,000 horses, and tons of equipment and supplies. An English observer remarked that it was the "stride of a giant." The army's advance from Fort Monroe up the Virginia Peninsula proved to be slow. McClellan's plan for a rapid seizure of Yorktown was foiled when he discovered that the Confederates had fortified a line across the Peninsula, causing him to decide on a siege of the city, which required considerable preparation.

McClellan continued to believe intelligence reports that credited the Confederates with two or three times the men they actually had. Early in the campaign, Confederate General John B. "Prince John" Magruder defended the Peninsula against McClellan's advance with a vastly smaller force. He created a false impression of many troops behind the lines and of even more troops arriving. He accomplished this by marching small groups of men repeatedly past places where they could be observed at a distance or were just out of sight, accompanied by great noise and fanfare. During this time, General Johnston was able to provide Magruder with reinforcements, but even then there were far fewer troops than McClellan believed were opposite him.

After a month of preparation, just before he was to assault the Confederate works at Yorktown, McClellan learned that Johnston had withdrawn up the Peninsula towards Williamsburg. McClellan was thus required to give chase without any benefit of the heavy artillery so carefully amassed in front of Yorktown. The Battle of Williamsburg on May 5 is considered a Union victory — McClellan's first — but the Confederate army was not destroyed and a bulk of their troops were successfully moved past Williamsburg to Richmond's outer defenses while it was waged, and over the next several days. McClellan had also placed hopes on a simultaneous naval approach to Richmond via the James River. That approach failed following the Union Navy's defeat at the Battle of Drewry's Bluff, about 7 miles (11 km) downstream from the Confederate capital, on May 15. Basing artillery on a strategic bluff high above a bend in the river, and sinking boats to create an impassable series of obstacles in the river itself, the Confederates had effectively blocked this potential approach to Richmond. McClellan's army cautiously inched towards Richmond over the next three weeks, coming to within four miles (6 km) of it. He established a supply base on the Pamunkey River (a navigable tributary of the York River) at White House Landing where the Richmond and York River Railroad extending to Richmond crossed, and commandeered the railroad, transporting steam locomotives and rolling stock to the site by barge.

On May 31, as McClellan planned an assault, his army was surprised by a Confederate attack. Johnston saw that the Union army was split in half by the rain-swollen Chickahominy River and hoped to defeat it in detail at Seven Pines and Fair Oaks. McClellan was unable to command the army personally because of a recurrence of malarial fever, but his subordinates were able to repel the attacks. Nevertheless, McClellan received criticism from Washington for not counterattacking, which some believed could have opened the city of Richmond to capture. Johnston was wounded in the battle, and General Robert E. Lee assumed command of the Army of Northern Virginia. McClellan spent the next three weeks repositioning his troops and waiting for promised reinforcements, losing valuable time as Lee continued to strengthen Richmond's defenses.

At the end of June, Lee began a series of attacks that became known as the Seven Days Battles. The first major battle, at Mechanicsville, was poorly coordinated by Lee and his subordinates and caused heavy casualties for little tactical gain. But the battle had significant impact on McClellan's nerve. The surprise appearance of Maj. Gen. Stonewall Jackson's troops in the battle (when they had last been reported to be many miles away in the Shenandoah Valley) convinced McClellan that he was even more significantly outnumbered than he had assumed. (He reported to Washington that he faced 200,000 Confederates, but there were actually 85,000.) As Lee continued his offensive at Gaines' Mill to the east, McClellan played a passive role, taking no initiative and waiting for events to unfold. He kept two thirds of his army out of action, fooled again by Magruder's theatrical diversionary tactics. That night, he decided to withdraw his army to a safer base, well below Richmond, on a portion of the James River that was under control of the Union Navy. In doing so, he may have unwittingly saved his army. Lee had assumed that the Union army would withdraw to the east toward its existing supply base and McClellan's move to the south delayed Lee's response for at least 24 hours. But McClellan was also tacitly acknowledging that he would no longer be able to invest Richmond, the object of his campaign; the heavy siege artillery required would be almost impossible to transport without the railroad connections available from his original supply base on the York River. In a telegram to Secretary of War Edwin Stanton, reporting on these events, McClellan blamed the Lincoln administration for his reversals. "If I save this army now, I tell you plainly I owe no thanks to you or to any other persons in Washington. You have done your best to sacrifice this army." Fortunately for McClellan's immediate career, Lincoln never saw that inflammatory statement (at least at that time) because it was censored by the War Department telegrapher.

McClellan was also fortunate that the failure of the campaign left his army mostly intact, because he was generally absent from the fighting and neglected to name a second-in-command to control his retreat. Military historian Stephen W. Sears wrote, "When he deserted his army on the Glendale and Malvern Hill battlefields during the Seven Days, he was guilty of dereliction of duty. Had the Army of the Potomac been wrecked on either of these fields (at Glendale the possibility had been real), that charge under the Articles of War would likely have been brought against him." (During Glendale, McClellan was five miles (8 km) away behind Malvern Hill, without telegraph communications and too distant to command the army. During the battle of Malvern Hill, he was on a gunboat, the U.S.S. Galena, which at one point was ten miles (16 km) away down the James River. During both battles, effective command of the army fell to his friend and V Corps commander Brigadier General Fitz John Porter. When the public heard about the Galena, it was yet another enormous embarrassment, comparable to the Quaker Guns at Manassas. Editorial cartoons during the 1864 presidential campaign would lampoon McClellan for preferring the safety of a ship while a battle was fought in the distance.)

McClellan was reunited with his army at Harrison's Landing on the James. Debates were held as to whether the army should be evacuated or attempt to resume an offensive toward Richmond. McClellan maintained his estrangement from Abraham Lincoln by his continuous call for reinforcements and by writing a lengthy letter in which he proposed strategic and political guidance for the war, continuing his opposition to abolition or seizure of slaves as a tactic. He concluded by implying he should be restored as general in chief, but Lincoln responded by naming Maj. Gen. Henry W. Halleck to the post without consulting, or even informing, McClellan. Lincoln and Stanton also offered command of the Army of the Potomac to Maj. Gen. Ambrose Burnside, who refused the appointment.

Back in Washington, a reorganization of units created the Army of Virginia under Maj. Gen. John Pope, who was directed to advance towards Richmond from the northeast. McClellan resisted calls to reinforce Pope's army and delayed return of the Army of the Potomac from the Peninsula enough so that the reinforcements arrived while the Northern Virginia Campaign was already underway. He wrote to his wife before the battle, "Pope will be thrashed ... & be disposed of [by Lee]. ... Such a villain as he is ought to bring defeat upon any cause that employs him." Lee had assessed McClellan's offensive nature and gambled on removing significant units from the Peninsula to attack Pope, who was beaten decisively at Second Bull Run in August. After the defeat of Pope at Second Bull Run, President Lincoln reluctantly returned to the man who had mended a broken army before. He realized that McClellan was a strong organizer and a skilled trainer of troops, able to recombine the units of Pope's army with the Army of the Potomac faster than anyone. On September 2, 1862, Lincoln named McClellan to command "the fortifications of Washington, and all the troops for the defense of the capital." The appointment was controversial in the Cabinet, a majority of whom signed a petition declaring to the president "our deliberate opinion that, at this time, it is not safe to entrust to Major General McClellan the command of any Army of the United States." The president admitted that it was like "curing the bite with the hair of the dog." But Lincoln told his secretary, John Hay, "We must use what tools we have. There is no man in the Army who can man these fortifications and lick these troops of ours into shape half as well as he. If he can't fight himself, he excels in making others ready to fight."

Northern fears of a continued offensive by Robert E. Lee were realized when he launched his Maryland Campaign on September 4, hoping to arouse pro-Southern sympathy in the slave state of Maryland. McClellan's pursuit began on September 5. He marched toward Maryland with six of his reorganized corps, about 84,000 men, while leaving two corps behind to defend Washington. McClellan's reception in Frederick, Maryland, as he marched towards Lee's army, was described by the correspondent for Harper's Magazine:

The General rode through the town on a trot, and the street was filled six or eight deep with his staff and guard riding on behind him. The General had his head uncovered, and received gracefully the salutations of the people. Old ladies and men wept for joy, and scores of beautiful ladies waved flags from the balconies of houses upon the street, and their joyousness seemed to overcome every other emotion. When the General came to the corner of the principal street the ladies thronged around him. Bouquets, beautiful and fragrant, in great numbers were thrown at him, and the ladies crowded around him with the warmest good wishes, and many of them were entirely overcome with emotion. I have never witnessed such a scene. The General took the gentle hands which were offered to him with many a kind and pleasing remark, and heard and answered the many remarks and compliments with which the people accosted him. It was a scene which no one could forget — an event of a lifetime.

Lee divided his forces into multiple columns, spread apart widely as he moved into Maryland and also maneuvered to capture the federal arsenal at Harpers Ferry. This was a risky move for a smaller army, but Lee was counting on his knowledge of McClellan's temperament. He told one of his generals, "He is an able general but a very cautious one. His army is in a very demoralized and chaotic condition, and will not be prepared for offensive operations — or he will not think it so — for three or four weeks. Before that time I hope to be on the Susquehanna." This was not a completely accurate assessment, but McClellan's army was moving lethargically, averaging only 6 miles (9.7 km) a day. However, Little Mac soon received a miraculous break of fortune. Union soldiers accidentally found a copy of Lee's orders that divided his army and delivered them to McClellan's headquarters in Frederick on September 13. Upon realizing the intelligence value of this discovery, McClellan threw up his arms and exclaimed, "Now I know what to do!" He waved the order at his old Army friend, Brig. Gen. John Gibbon, and said, "Here is a paper with which if I cannot whip Bobbie Lee, I will be willing to go home." He telegraphed President Lincoln: "I have the whole rebel force in front of me, but I am confident, and no time shall be lost. I think Lee has made a gross mistake, and that he will be severely punished for it. I have all the plans of the rebels, and will catch them in their own trap if my men are equal to the emergency. ... Will send you trophies.".

Despite this show of bravado, McClellan continued his cautious line. After telegraphing to the president at noon on September 13, rather than ordering his units to set out for the South Mountain passes immediately, he ordered them to depart the following morning. The 18 hours of delay allowed Lee time to react, because he received intelligence from a Confederate sympathizer that McClellan knew of his plans. (The delay also doomed the federal garrison at Harpers Ferry because the relief column McClellan sent could not reach them before they surrendered to Stonewall Jackson.) In the Battle of South Mountain, McClellan's army was able to punch through the defended passes that separated them from Lee, but also gave Lee enough time to concentrate many of his men at Sharpsburg, Maryland. The Battle of South Mountain presented McClellan with an opportunity for one of the great theatrical moments of his career, as historian Sears describes:

The mountain ahead was wreathed in smoke eddies of battle smoke in which the gun flashes shone like brief hot sparks. The opposing battle lines on the heights were marked by heavier layers of smoke, and columns of Federal troops were visible winding their way up the mountainside, each column ... looking like a 'monstrous, crawling, blue - black snake' ... McClellan posed against this spectacular backdrop, sitting motionless astride his warhorse Dan Webster with his arm extended, pointing Hooker's passing troops toward the battle. The men cheered him until they were hoarse ... and some broke ranks to swarm around the martial figure and indulge in the 'most extravagant demonstrations'.

The Union army reached Antietam Creek, to the east of Sharpsburg, on the evening of September 15. A planned attack on September 16 was put off because of early morning fog, allowing Lee to prepare his defenses with an army less than half the size of McClellan's. The Battle of Antietam on September 17, 1862, was the single bloodiest day in American military history. The outnumbered Confederate forces fought desperately and well. Despite significant advantages in manpower, McClellan was unable to concentrate his forces effectively, which meant that Lee was able to shift his defenders to parry each of three Union thrusts, launched separately and sequentially against the Confederate left, center, and finally the right. And McClellan was unwilling to employ his ample reserve forces to capitalize on localized successes. Historian James M. McPherson has pointed out that the two corps McClellan kept in reserve were in fact larger than Lee's entire force. The reason for McClellan's reluctance was that, as in previous battles, he was convinced he was outnumbered.

The battle was tactically inconclusive, although Lee technically was defeated because he withdrew first from the battlefield and retreated back to Virginia. McClellan wired to Washington, "Our victory was complete. The enemy is driven back into Virginia." Yet there was obvious disappointment that McClellan had not crushed Lee, who was fighting with a smaller army with its back to the Potomac River. Although McClellan's subordinates can claim their share of responsibility for delays (such as Ambrose Burnside's misadventures at Burnside Bridge) and blunders (Edwin V. Sumner's attack without reconnaissance), these were localized problems from which the full army could have recovered. As with the decisive battles in the Seven Days, McClellan's headquarters were too far to the rear to allow his personal control over the battle. He made no use of his cavalry forces for reconnaissance. He did not share his overall battle plans with his corps commanders, which prevented them from using initiative outside of their sectors. And he was far too willing to accept cautious advice about saving his reserves, such as when a significant breakthrough in the center of the Confederate line could have been exploited, but Fitz John Porter is said to have told McClellan, "Remember, General, I command the last reserve of the last Army of the Republic."

Despite being a tactical draw, Antietam is considered a turning point of the war and a victory for the Union because it ended Lee's strategic campaign (his first invasion of the North) and it allowed President Lincoln to issue the Emancipation Proclamation on September 22, taking effect on January 1, 1863. Although Lincoln had intended to issue the proclamation earlier, he was advised by his Cabinet to wait until a Union victory to avoid the perception that it was issued out of desperation. The Union victory and Lincoln's proclamation played a considerable role in dissuading the governments of France and Britain from recognizing the Confederacy; some suspected they were planning to do so in the aftermath of another Union defeat. McClellan had no prior knowledge that the plans for emancipation rested on his battle performance.

When McClellan failed to pursue Lee aggressively after Antietam, Lincoln ordered that he be removed from command on November 5. Maj. Gen. Ambrose Burnside assumed command of the Army of the Potomac on November 7. McClellan wrote to his wife, "Those in whose judgment I rely tell me that I fought the battle splendidly and that it was a masterpiece of art. ... I feel I have done all that can be asked in twice saving the country. ... I feel some little pride in having, with a beaten & demoralized army, defeated Lee so utterly. ... Well, one of these days history will I trust do me justice."

Secretary Stanton ordered McClellan to report to Trenton, New Jersey, for further orders, although none were issued. As the war progressed, there were various calls to return Little Mac to an important command, following the Union defeats at Fredericksburg and Chancellorsville, as Robert E. Lee moved north at the start of the Gettysburg Campaign, and as Jubal Early threatened Washington in 1864. When Ulysses S. Grant became general in chief, he discussed returning McClellan to an unspecified position. But all of these opportunities were impossible, given the opposition within the administration and the knowledge that McClellan posed a potential political threat. McClellan worked for months on a lengthy report describing his two major campaigns and his successes in organizing the Army, replying to his critics and justifying his actions by accusing the administration of undercutting him and denying him necessary reinforcements. The War Department was reluctant to publish his report because, just after completing it in October 1863, McClellan openly declared his entrance to the political stage as a Democrat.

McClellan was nominated by the Democrats to run against Abraham Lincoln in the 1864 U.S. presidential election. Following the example of Winfield Scott, he ran as a U.S. Army general still on active duty; he did not resign his commission until election day, November 8, 1864. He supported continuation of the war and restoration of the Union, but the party platform, written by Copperhead Clement Vallandigham of Ohio, was opposed to this position. The platform called for an immediate cessation of hostilities and a negotiated settlement with the Confederacy. McClellan was forced to repudiate the platform, which made his campaign inconsistent and difficult. He also was not helped by the party's choice for vice president, George H. Pendleton, a peace candidate from Ohio.

The deep division in the party, the unity of the Republicans (running under the label "National Union Party"), and the military successes by Union forces in the fall of 1864 doomed McClellan's candidacy. Lincoln won the election handily, with 212 Electoral College votes to 21 and a popular vote of 403,000, or 55%. While McClellan was highly popular among the troops when he was commander, they voted for Lincoln over him by margins of 3-1 or higher. Lincoln's share of the vote in the Army of the Potomac was 70%.

After the war, McClellan and his family departed for a lengthy trip to Europe (from 1865 to 1868), during which he did not participate in politics. When he returned, the Democratic Party expressed some interest in nominating him for president again, but when it became clear that Ulysses S. Grant would be the Republican candidate, this interest died. McClellan worked on engineering projects in New York City and was offered the position of president of the newly formed University of California. McClellan was appointed chief engineer of the New York City Department of Docks in 1870. Evidently the position did not demand his full-time attention because, starting in 1872, he also served as the president of the Atlantic and Great Western Railroad. He and his family returned to Europe from 1873 to 1875. In March 1877, McClellan was nominated by Governor Lucius Robinson to be the first Superintendent of Public Works but was rejected by the New York State Senate as being "incompetent for the position."

In 1877, McClellan was nominated by the Democrats for Governor of New Jersey, an action that took him by surprise because he had not expressed an interest in the position. He was elected and served a single term from 1878 to 1881, a tenure marked by careful, conservative executive management and minimal political rancor. The concluding chapter of his political career was his strong support in 1884 for the election of Grover Cleveland. He hoped to be named secretary of war in Cleveland's cabinet, a position for which he was well suited, but political rivals from New Jersey were able to block his nomination.

McClellan's final years were devoted to traveling and writing. He justified his military career in McClellan’s Own Story, published posthumously in 1887. He died unexpectedly at age 58 at Orange, New Jersey, after having suffered from chest pains for a few weeks. His final words, at 3 a.m., October 29, 1885, were, "I feel easy now. Thank you." He is buried at Riverview Cemetery, Trenton, New Jersey.

McClellan's son, George B. McClellan, Jr. (1865 – 1940), was born in Dresden, Germany, during the family's first trip to Europe. Known within the family as Max, he was also a politician, serving as a United States Representative from New York State and as Mayor of New York City from 1904 to 1909. McClellan's daughter, Mary ("May") (1861 – 1945), married a French diplomat and spent much of her life abroad. His wife Ellen died in Nice, France, while visiting May at "Villa Antietam." Neither Max nor May gave the McClellans any grandchildren.

The New York Evening Post commented in McClellan's obituary, "Probably no soldier who did so little fighting has ever had his qualities as a commander so minutely, and we may add, so fiercely discussed." This fierce discussion has continued for over a century. McClellan is usually ranked in the lowest tier of Civil War generals. However, the debate over McClellan's ability and talents remains the subject of much controversy among Civil War and military historians. He has been universally praised for his organizational abilities and for his very good relations with his troops. They referred to him affectionately as "Little Mac"; others sometimes called him the "Young Napoleon". It has been suggested that his reluctance to enter battle was caused in part by an intense desire to avoid spilling the blood of his men. Ironically, this led to failing to take the initiative against the enemy and therefore passing up good opportunities for decisive victories, which could have ended the war early, and thereby could have spared thousands of soldiers who died in those subsequent battles. Generals who proved successful in the war, such as Lee and Grant, tended to be more aggressive and more willing to risk a major battle even when all preparations were not perfect. McClellan himself summed up his cautious nature in a draft of his memoirs:

It has always been my opinion that the true course in conducting military operations, is to make no movement until the preparations are as complete as circumstances permit, & never to fight a battle without some definite object worth the probable loss.

McClellan's reluctance to press his enemy aggressively was probably not a matter of personal courage, which he demonstrated well enough by his bravery under fire in the Mexican - American War. Stephen Sears wrote,

There is indeed ample evidence that the terrible stresses of commanding men in battle, especially the beloved men of his beloved Army of the Potomac, left his moral courage in tatters. Under the pressure of his ultimate soldier's responsibility, the will to command deserted him. Glendale and Malvern Hill found him at the peak of his anguish during the Seven Days, and he fled those fields to escape the responsibility. At Antietam, where there was nowhere for him to flee to, he fell into a paralysis of indecision. Seen from a longer perspective, General McClellan could be both comfortable and successful performing as executive officer, and also, if somewhat less successfully, as grand strategist; as battlefield commander, however, he was simply in the wrong profession.

One of the reasons that McClellan's reputation has suffered is because of his own memoirs. Historian Allan Nevins wrote, "Students of history must always be grateful McClellan so frankly exposed his own weaknesses in this posthumous book." Doris Kearns Goodwin claims that a review of his personal correspondence during the war reveals a tendency for self-aggrandizement and unwarranted self-congratulation. His original draft was completed in 1881, but the only copy was destroyed by fire. He began to write another draft of what would be published posthumously, in 1887, as McClellan's Own Story. However, he died before it was half completed and his literary executor, William C. Prime, editor of the pro-McClellan New York Journal of Commerce, included excerpts from some 250 of McClellan's wartime letters to his wife, in which it had been his habit to reveal his innermost feelings and opinions in unbridled fashion.

Robert E. Lee, on being asked (by his cousin, and recorded by his son) who was the ablest general on the Union side during the late war, replied emphatically: "McClellan, by all odds!"

While McClellan's reputation has suffered over time, especially over the last 75 years, there is a small but intense cadre of American Civil War historians who believe that the general has been poorly served on at least four levels. First, McClellan proponents say that because the general was a conservative Democrat with great personal charisma, radical Republicans fearing his political potential deliberately undermined his field operations. Second, that as the radical Republicans were the true winners coming out of the American Civil War, they were able to write its history, placing their principal political rival of the time, McClellan, in the worst possible light. Third, that historians eager to jump on the bandwagon of Lincoln as America's greatest political icon worked to outdo one another in shifting blame for early military failures from Lincoln and Secretary of War Edwin M. Stanton to McClellan. And fourth, that Lincoln and Stanton deliberately undermined McClellan because of his conciliatory stance towards the South, which might have resulted in a less destructive end to the war had Richmond fallen as a result of the Peninsula Campaign. Proponents of this school claim that McClellan is criticized more for his admittedly abrasive personality than for his actual field performance.

Several geographic features and establishments have been named for George B. McClellan. These include Fort McClellan in Alabama, McClellan Butte in the Mount Baker - Snoqualmie National Forest, where he traveled while conducting the Pacific Railroad Survey in 1853, McClellan Street in North Bend, Washington, McClellan Street in South Philadelphia, McClellan Road in Cupertino, California, McClellan Elementary School in Chicago, and a bronze equestrian statue honoring General McClellan in Washington, D.C. Another equestrian statue honors him in front of Philadelphia City Hall.