Dennis Hart Mahan was the child of Irish Catholic immigrants. He was baptized at New York City, though like other immigrant children of that era, e.g., General Philip Sheridan, it is unclear whether Mahan was born at New York or in Ireland. Mahan graduated from West Point in 1824, first in his class. Such was his acumen that in his third year he was appointed acting assistant professor of mathematics. After graduation, he started teaching at the Military Academy with the very next class of cadets (1824). In 1826, he was sent to Europe to study advanced engineering techniques and military institutions. Upon returning to West Point in 1830, he was promoted to Professor of Civil and Military Engineering. He resigned his commission in 1832 to accept the position of Chair of the Engineering Department.

An important influence on the military conduct of the American Civil War and Civil Engineering, Mahan is best understood as an educator and technology transfer agent, not a theorist. Mahan almost single - handedly compiled and transferred the best of European engineering to the United States and other English speaking parts of the world. Virtually all 19th century American engineering schools were started with West Point educated faculty or adopted its texts.

As a professor of military science at West Point, in addition to engineering methodology, Mahan promoted the development of professionalism and wrote extensively on fixed fortifications, field fortifications, strategy and tactics. His books on military thought were widely influential. His writings became standard textbooks in the worldwide field from the time they were written until after World War I. Mahan also founded the Napoleon Seminar at West Point, where advanced undergraduates and senior officers including Lee, Reynolds, Thomas and McClellan, studied and discussed the great European wars, Napoleon and Frederick the Great.

Professor Mahan's lectures and writings about military fortifications and strategy were instrumental in the conduct of the Civil War by the officers on both sides. Most of the Civil War commanders, whether Union or Confederate, learned about entrenchment, fortifications, and how to conduct warfare in the classes that he taught at West Point, and from his pre - Civil War writings.

While the influence of Mahan and West Point on the Civil War remains controversial and neglected, it is not difficult to find many of the Civil War's tactics and strategies presaged in Mahan's work. Topography and the use of terrain to advantage became important thanks to Mahan, as well as the principles of concentration, "celerity" and calculated risk. Lee used the Blue Ridge Mountains to screen his invasions of Pennsylvania and Maryland. Grant used rivers to screen his approaches to Vicksburg and Richmond. The French called this envelopment strategy "la manoeuvre sur les derrières." At Vicksburg, after the envelopment, Grant used a strategy of the central position to defeat his opponent in detail, a Napoleon tactic well known to Mahan's admirer and Grant's lieutenant WT Sherman, if not Grant himself. At Gettysburg, Reynolds saw the opportunity for a Corps d'Armee engagement where his leading Corps held off Confederate attacks while the mass of the Union army concentrated. The Union's disposition on Cemetery Ridge at Gettysburg is almost perfectly sited according to Mahan's recommendations in "Field Fortifications" for disrupting an opponent's artillery fire. Meade's inability to trap Lee after Gettysburg certainly had something to do with Lee heeding Mahan's advice on planning for a protected line of retreat. Perhaps the Union and Confederate armies would have adopted flexible and resilient division and corps formations in any event, but some credit for the adoption of these extremely important military formations belongs to West Point and Mahan and their contribution to the professional development of most of the Civil War's important officers.

Mahan Hall at the United States Military Academy at West Point was named in his honor. Built in 1971, it houses the Academy's Departments of Civil & Mechanical Engineering (CME) and Systems Engineering. Mahan Hall at the U.S. Naval Academy and the Naval War College were named for his son, Alfred Thayer Mahan.

Mahan's writings include:

• Treatise on Field Fortifications (1836)
• Elementary Course of Civil Engineering (1837; revised 1868)
• Elementary Treatise on Advanced Guard, Outposts, and Detachment Service of Troops (1847; revised, 1862)
• Summary on the Cause of Permanent Fortifications and of the Attack and Defense of Permanent Works (1850)
• Elementary Treatise on Industrial Drawing (1853)
• Editor, with additions, the American edition of Mosely's Mechanical Principles of Engineering and Architecture (1856)
• Descriptive Geometry, as applied to the Drawing of Fortifications and Stereometry (1864)
• An Elementary Course on Military Engineering [covering] Field Fortifications, Military Mining, and Siege Operations (1865)
• Permanent Fortifications (1867)

Alfred Thayer Mahan (September 27, 1840 – December 1, 1914) was a United States Navy flag officer, geostrategist, and historian, who has been called "the most important American strategist of the nineteenth century." His concept of "sea power" was based on the idea that countries with greater naval power will have greater worldwide impact; it was most famously presented in The Influence of Sea Power Upon History, 1660 – 1783 (1890). The concept had an enormous influence in shaping the strategic thought of navies across the world, especially in the United States, Germany, Japan and Britain, ultimately causing the World War I naval arms race. His ideas still permeate the U.S. Navy Doctrine.

Several ships were named USS Mahan, including the lead vessel of a class of destroyers.

Born at West Point, New York, to Dennis Hart Mahan (a professor at the United States Military Academy) and Mary Helena Mahan, he attended Saint James School, an Episcopal college preparatory academy in western Maryland. He then studied at Columbia for two years where he was a member of the Philolexian Society debating club and then, against his parents' wishes, transferred to the Naval Academy, where he graduated second in his class in 1859.

Commissioned as a Lieutenant in 1861, Mahan served the Union in the American Civil War as an officer on USS Worcester, Congress, Pocahontas, and James Adger, and as an instructor at the Naval Academy. In 1865 he was promoted to Lieutenant Commander, and then to Commander (1872), and Captain (1885). As commander of the USS Wachusett he was stationed at Callao, Peru, protecting American interests during the final stages of the War of the Pacific.

Despite his professed success in the Navy, his skills in actual command of a ship were not exemplary, and a number of vessels under his command were involved in collisions, with both moving and stationary objects. He had an affection for old square - rigged vessels, and did not like smoky, noisy steamships of his time; he tried to avoid active sea duty. On the other hand, the books he wrote ashore made him arguably the most influential naval historian of the period. In pointing out how unlikely his ascent was, Kyle Whitney compared his chances of achieving prominence in the navy to that of "a cheerleader becoming president".

In 1885, he was appointed lecturer in naval history and tactics at the Naval War College. Before entering on his duties, College President Rear Admiral Stephen B. Luce pointed Mahan in the direction of writing his future studies on the influence of sea power. For his first year on the faculty, he remained at his home in New York City researching and writing his lectures. Upon completion of this research period, he was to succeed Luce as President of the Naval War College from June 22, 1886 to January 12, 1889 and again from July 22, 1892 to May 10, 1893. There, in 1887, he met and befriended Theodore Roosevelt, then a visiting lecturer, who would later become president of the United States.

Mahan plunged into the library and wrote lectures that drew heavily on standard classics and the ideas of work of Henri Jomini. The lectures became his sea power studies: The Influence of Sea Power upon History, 1660 – 1783 (1890); The Influence of Sea Power upon the French Revolution and Empire, 1793 – 1812 (2 vols., 1892); and Sea Power in Relation to the War of 1812 (2 vols., 1905). The Life of Nelson: The Embodiment of the Sea Power of Great Britain (2 vols., 1897) supplemented the series. Mahan stresses the importance of the individual in shaping history, and extols the traditional values of loyalty, courage, and service to the state. Mahan sought to resurrect Horatio Nelson as a national hero in Britain and used the book as a platform for expressing his views on naval strategy and tactics. Criticisms of the work focused on Mahan's handling of Nelson's love affair with Lady Emma Hamilton, but it remains the standard biography. In addition to these works, Mahan wrote more than a hundred articles on international politics and related topics, which were closely read by policy makers.

Upon being published, Mahan struck up a friendship with pioneering British naval historian Sir John Knox Laughton, the pair maintaining this relationship through correspondence and visits when Mahan was in London. Mahan was later described as a 'disciple' of Laughton, although the two men were always at pains to distinguish between each other's line of work, Laughton seeing Mahan as a theorist while Mahan called Laughton 'the historian'.

Mahan's views were shaped by the seventeenth century conflicts between Holland, England, France and Spain, and by the nineteenth century naval wars between France and Britain, where British naval superiority eventually defeated France, consistently preventing invasion and blockade (Napoleonic war: Battle of Trafalgar and Continental System). To a modern reader, the emphasis on controlling seaborne commerce is commonplace, but in the nineteenth century, the notion was radical, especially in a nation entirely obsessed with expansion on to the continent's western land. On the other hand, Mahan's emphasis of sea power as the crucial fact behind Britain's ascension neglected the well documented roles of diplomacy and armies; Mahan's theories could not explain the success of terrestrial empires, such as Bismarckian Germany. However, as the Royal Navy's blockade of the German Empire was a critical direct and indirect factor in the eventual German collapse, Mahan's theories were vindicated by the First World War.

In the context of his time, Mahan backed a revival of Manifest Destiny through overseas imperialism. He held that sea power would require the United States to acquire defensive bases in the Caribbean and Pacific as well as take possession of Hawaii. This came at the time when the United States launched a major shipbuilding program to move the United States to the third place amongst worldwide naval powers by 1900.

Mahan used history as a stock of lessons to be learned — or more exactly, as a pool of examples that exemplified his theories. Mahan believed that national greatness was inextricably associated with the sea, with its commercial usage in peace and its control in war. His goal was to discover the laws of history that determined who controlled the seas. His theoretical framework came from Jomini, with an emphasis on strategic locations (such as chokepoints, canals, and coaling stations), as well as quantifiable levels of fighting power in a fleet. The primary mission of a navy was to secure the command of the sea. This not only permitted the maintenance of sea communications for one's own ships while denying their use to the enemy but also, if necessary, provided the means for close supervision of neutral trade. This control of the sea could not be achieved by destruction of commerce but only by destroying or neutralizing the enemy fleet. This called for concentration of naval forces composed of capital ships, not unduly large but numerous, well manned with crews thoroughly trained, and operating under the principle that the best defense is an aggressive offense.

Mahan contended that with command of the sea, even if local and temporary, naval operations in support of land forces can be of decisive importance and that naval supremacy can be exercised by a transnational consortium acting in defense of a multinational system of free trade. His theories — written before the submarine became a factor in warfare against shipping — delayed the introduction of convoys as a defense against German U-Boats in World War I. By the 1930s the U.S. Navy was building long range submarines to raid Japanese shipping, but the Japanese, still tied to Mahan, designed their submarines as ancillaries to the fleet and failed to attack American supply lines in the Pacific in World War II.

Mahan argued that radical technological change does not eliminate uncertainty from the conduct of war, and therefore a rigorous study of history should be the basis of naval officer education.

Sumida (2000) argues Mahan believed that good political and naval leadership was no less important than geography when it came to the development of sea power. Second, his unit of political analysis insofar as sea power was concerned was a transnational consortium rather than the single nation - state. Third, his economic ideal was free trade rather than autarchy. Fourth, his recognition of the influence of geography on strategy was tempered by a strong appreciation of the power of contingency to affect outcomes.

Mahan prepared a secret contingency plan of 1890 in case war should break out between Britain and the United States. Mahan concluded that the British would attempt to blockade the eastern ports, so the American Navy should be concentrated in one of these ports, preferably New York with its two widely separated exits, while torpedo boats should defend the other harbors. This concentration of the U.S. fleet would force the British to tie down such a large proportion of their navy to watch the New York exits that the other American ports would be relatively safe. Detached American cruisers should wage "constant offensive action" against the enemy's exposed positions, and if the British were to weaken their blockade force off New York to attack another American port, the concentrated U.S. fleet should seize the opportunity to escort an invasion fleet to capture the British coaling ports in Nova Scotia, thereby seriously weakening the British ability to engage in naval operations off the American coast. This contingency plan is a clear example of the application of Mahan's principles of naval war, with a clear reliance on Jomini's principle of controlling strategic points.

Mahan was a frequent commentator on world naval, strategic and diplomatic affairs. In the 1890s he argued that the United States should concentrate its naval fleet and obtain Hawaii as a hedge against Japanese eastward expansion and that the U.S. should help maintain a balance of power in the region in order to advance the principle of the Open Door policy both commercially and culturally. Mahan represented the U.S. at the first international conference on arms control that was initiated by Russia in 1899. Russia sought a "freeze" to keep from falling behind in Europe's arms race. Other countries attended in order to mollify various peace groups. No significant arms limitations agreements were reached. A proposal on neutral trade rights was debated but ruled out of order by the Russians. The only significant result of the conference was the establishment of an ineffective Permanent Court of Arbitration at the Hague.

Timeliness contributed no small part to the widespread acceptance and resultant influence of Mahan's views. Although his history was relatively thin (he relied on secondary sources), the vigorous style and clear theory won widespread acceptance of navalists across the world. Sea power supported the new colonialism which was asserting itself in Africa and Asia. Given the very rapid technological changes underway in propulsion (from coal to oil, from boilers to turbines), ordnance (with better fire directors, and new high explosives) and armor and emergence of new craft such as destroyers and submarines, Mahan's emphasis on the capital ship and the command of the sea came at an opportune moment.

Mahan's name became a household word in the German navy, as Kaiser William II ordered his officers to read Mahan, and Admiral Alfred von Tirpitz (1849 – 1930) used Mahan's reputation to finance a powerful surface fleet.

Between 1890 and 1915, Mahan and British admiral John Fisher (1841 – 1920) faced the problem of how to dominate home waters and distant seas with naval forces not strong enough to do both. Mahan argued for a universal principle of concentration of powerful ships in home waters and minimized strength in distant seas, while Fisher reversed Mahan by utilizing technological change to propose submarines for defense of home waters and mobile battle cruisers for protection of distant imperial interests.

The French were less susceptible to Mahan's theories. French naval doctrine in 1914 was dominated by Mahan's theory of sea power and therefore geared toward winning decisive battles and gaining mastery of the seas. But the course of World War I changed ideas about the place of the navy, as the refusal of the German fleet to engage in a decisive battle, the Dardanelles expedition of 1915, the development of submarine warfare, and the organization of convoys all showed the navy's new role in combined operations with the army. The navy's part in securing victory was not fully understood by French public opinion in 1918, but a synthesis of old and new ideas arose from the lessons of the war, especially by admiral Raoul Castex (1878 – 1968), from 1927 to 1935, who synthesized in his five - volume Théories Stratégiques the classical and materialist schools of naval theory. He reversed Mahan's theory that command of the sea precedes maritime communications and foresaw the enlarged roles of aircraft and submarines in naval warfare. Castex enlarged strategic theory to include nonmilitary factors (policy, geography, coalitions, public opinion, and constraints) and internal factors (economy of force, offense and defense, communications, operational plans, morale, and command) to conceive a general strategy to attain final victory.

Ideologically, the United States Navy initially opposed replacing its sailing ships with steam powered ships after the Civil War; Mahan argued that only a fleet of armored battleships might be decisive in a modern war. According to the decisive battle doctrine, a fleet must not be divided; Mahan's work encouraged technological improvement in convincing opponents that naval knowledge and strategy remained necessary, but that domination of the seas dictated the necessity of the speed and predictability of the steam engine.

His books were greatly acclaimed, and closely studied in Britain and Imperial Germany, influencing the build up of their forces prior to the First World War. Mahan influenced the naval portion of the Spanish - American War, and the battles of Tsushima, Jutland, and the Atlantic. His work influenced the doctrines of every major navy in the interwar period.

Mahan's concept of sea power extended beyond naval superiority; that in peace time, states should increase production and shipping capacities, acquire overseas possessions — either colonies or privileged access to foreign markets — yet stressed that the number of coal fuel stations and strategic bases should be few, not to drain too many resources from the mother country.

Although Mahan's influence on foreign powers has been generally recognized, only rather recently have scholars called attention to his role as significant in the growth of American overseas possessions, the rise of the new American navy, and the adoption of the strategic principles upon which it operated. He died in Washington a few months after the outbreak of World War I.

The Influence of Seapower Upon History, 1660 – 1783 was translated to Japanese and used as a textbook in the Imperial Japanese Navy (IJN). This strongly affected the IJN's Pacific War conduct, emphasizing the "decisive battle" doctrine — even at the expense of protecting trade.

The IJN's pursuit of the "decisive battle" was such that it contributed to Imperial Japan's defeat in 1945, and so rendered obsolete the doctrine of the decisive battle between fleets, because of the development of the submarine and the aircraft carrier. However, one could argue that the IJN did not adhere entirely to Mahan's doctrine, as they did divide their main force from time to time, particularly the extensive division of warships in a complicated battle plan that led to the disaster at Midway, and as such sealed their own defeat.

Between 1889 and 1892 Mahan was engaged in special service for the Bureau of Navigation, and in 1893 he was appointed to command the powerful new protected cruiser Chicago on a visit to Europe, where he was received and feted. He returned to lecture at the War College and then, in 1896, he retired from active service, returning briefly to duty in 1898 to consult on naval strategy for the Spanish - American War.

Mahan continued to write voluminously and received honorary degrees from Oxford, Cambridge, Harvard, Yale, Columbia, Dartmouth, and McGill.

In 1902 Mahan invented the term "Middle East", which he used in the article "The Persian Gulf and International Relations", published in September in the National Review.

He became Rear Admiral in 1906 by an act of Congress promoting all retired captains who had served in the Civil War. At the outbreak of World War I, he initially engaged in the cause of Great Britain, but an order of President Woodrow Wilson prohibited all active and retired officers from publishing comments on the war. Mahan died of heart failure on December 1, 1914.

• The United States Naval Academy's Mahan Hall was named in his honor, as was Mahan Hall at the Naval War College. Mahan Hall at the United States Military Academy was named for his father, Dennis Hart Mahan.
• A.T. Mahan Elementary School and A.T. Mahan High School at Keflavik Naval Air Station in Iceland, were also named in his honor.
• Naval Sea Cadet Corps unit in Albany, New York is honored with his name as the Mahan Division also named after the USS Mahan

• The Gulf and Inland Waters (1883)
• The Influence of Sea Power Upon History, 1660 – 1783 (1890)
  • The Influence of Sea Power Upon History, 1660 – 1805 (abridged ed, 1980)
  • The Influence of Sea Power Upon History, 1660 – 1783 (1890)
  • The Influence of Sea Power upon the French Revolution and Empire, 1793 – 1812 (1892) at archive.org
• Admiral Farragut (1892)
• The Future in Relation To American Naval Power, Harper's New Monthly Magazine, Oct 1895
• The Life of Nelson: The Embodiment of the Sea Power of Great Britain (1897)
• The Life of Nelson, Volume 1 (of 2) by A. T. Mahan at Project Gutenberg
• The Life of Nelson, Volume 2 (of 2) by A. T. Mahan at Project Gutenberg
• The Interest of America in Sea Power, Present and Future (1897)
• Lessons of the War with Spain, and Other Articles (1899)
• The Problem of Asia and Its Effect Upon International Policies (1900)
• Story of the War in South Africa 1899 - 1900 (1900)
  
• Types of Naval Officers Drawn from the History of the British Navy (1901)
• Sea Power in Its Relation to the War of 1812 (2 vols.) (1905) (Boston: Little Brown) American Library Association.
• Reflections, Historic and Other, Suggested by the Battle of the Japan Sea. (1906) Proceedings magazine, June 1906, United States Naval Institute.
• From Sail to Steam, Recollections of Naval life (1907)
• Naval Administration and Warfare: Some General Principles, with Other Essays (1908)
• The Harvest Within: Thoughts on the Life of the Christian (1909)
• Naval Strategy: Compared and Contrasted with the Principles and Practice of Military Operations on Land (1911)
• Armaments and Arbitration; or, The Place of Force in the International Relations of States (1912)
• The Major Operations of the Navies in the War of American Independence (1913) at Project Gutenberg
• The Harvest Within: Thoughts on the Life of the Christian (1909)
• The Major Operations of the Navies in the War of American Independence (1913)