The Wabash reached her first port of call, Gibraltar, on August 17, 1858. She cruised in the Mediterranean, and the cadet officers visited the cities of the old world accessible to them, often taking trips inland. Dewey was assigned to keep the ship's log. The Wabash returned to the New York Navy Yard on 16 December 1859 and decommissioned there on 20 December 1859. Dewey served on two short term cruises in 1860.

During the Civil War, the West Gulf Blockading Squadron, was engaged in the blockade of the mouths of the Mississippi River. At the end of 1861 Commander David D.Porter urged action upon the Navy Department. By that time the Confederates had formed immensely strong defenses along the river delta. The plan, which was put into operation in the spring of 1862, proposed a naval expedition, intended to reduce the fortifications near the mouth of the river, and to capture New Orleans, to be followed by an army under General Benjamin Franklin Butler which would then take possession of that city and region, after which the war vessels would proceed up the river, reduce the forts along its banks and co-operate with the gunboats already commanding the upper part of the valley, and later with the Union armies operating in Tennessee and northern Mississippi. This plan was ultimately carried out, but it required more time, cost of life and material, and combat than were anticipated; and it gave Dewey invaluable experience.

The defenses of the river consisted of two immensely strong forts, Jackson and St. Philip, on the banks nearly opposite one another and about midway between the mouth of the river and New Orleans. Farther up there was also a series of strong waterside batteries at Chalmette. In addition to this the Confederates had established a line of obstructions across the river below the forts. The defenses were such as it was supposed no naval expedition would try to attack. The attack was aided by Porter's preliminary bombardment to weaken the Confederate works. This was accomplished with mortar boats that fired a thirteen - inch shell. They were anchored under protection of the banks and forest some distance below the forts, and for many days rained upon them continual fire as to half destroy the fortifications, and overcome a large part of the Confederate garrisons. This conflict is known as the Battle of Forts Jackson and St. Philip.

The Capture of New Orleans followed. At the end of this preliminary bombardment an attempt was made to run past the forts and the Confederate vessels gathered near them. This was begun on April 24, 1862, the fleet moving forward in three divisions, the first under command of Captain Theodorus Bailey in the Cayuga, followed closely by the Pensacola (afterward under Dewey's command), and that by the Mississippi, in which Dewey was executive lieutenant. These big ships kept near the west bank where the current was weaker and the water deeper; but this brought them right under the muzzles of the guns of Fort St. Philip, which had been little damaged by the mortar boats. Dewey guided the Mississippi, in shallow water where he expected to run aground any moment, to a successful attack against the fortifications.

The Confederates had afloat there an iron covered ram called Manassas — a cigar - shaped craft, almost wholly submerged but the nose was a sharp iron prow, designed to pierce the hull, beneath the water line, of an enemy's ship. She had rushed down the river had struck at everything in her way. Appearing suddenly from behind the Pensacola, when that vessel was slowing up opposite Fort St. Philip to enable her men to fire more effectively into the faces of the garrison, she had made a rush for the Mississippi; but Dewey steered his helm so as to avoid her prow and escape. Then, her upper structure pierced with his shot, but her machinery uninjured, the ram continued and nearly destroyed both the Brooklyn and Hartford before she was driven away. Then she turned and ran up the river, in chase of Bailey's ships, which were leading the way toward New Orleans, and Farragut signaled to the Mississippi to run her down. He gave the order and the Mississippi attacked. But just as the Union vessel was to override it, it dodged the blow by a quick turn of the helm and ran ashore, where the crew deserted the stranded vessel. Commander Smith sent a boat's crew to set fire to it and when they had returned, it was scuttled with cannon fire.

Having got past the forts, the Mississippi sailed up the river with the leading ships, until they came to the Chalmette batteries, where they destroyed their garrisons; then the ship was sent back with some others to a waiting position near the forts, to protect the landing of Butler's troops. This was Dewey's first battle in which he distinguished himself. For the remainder of that year all that Farragut's fleet attempted to do was to patrol the lower river — a dangerous duty, for the banks swarmed with sharpshooters, lying in wait among the trees. Here and there, also, an interval of quiet, would give the Confederates an opportunity to erect a concealed battery. They had also a way of running two or three field guns up behind the natural breastworks afforded by the levee, and unexpectedly opening fire upon some ship passing near the shore, or lying at anchor.

At Port Hudson, Louisiana, the Confederates had been constructing and strengthening their second line of defense of the river valley, until they considered it impregnable. The national forces had been unable to prevent this but, when the spring campaign of 1863 began, it was so important for the river to be opened, that Farragut resolved to attempt to run by the Port Hudson batteries, if he could not demolish them. The whole fleet was arranged for this attempt on March 14, 1863, at midnight, when Dewey saw fiercer fighting than he ever saw again.

Port Hudson was a small town on the east bank of the Mississippi, 13 miles upriver from Baton Rouge, Louisiana, at a point where the river makes the passage there a subject of anxiety to pilots even in daylight and in time of peace. In the spring of 1863 a crescentic series of powerful fortifications, having a concentric field of fire, bordered the outside of the bend. The gunners were aided at night by the illumination of the water afforded by setting fire to huge beacons and rafts of pine knots; and had the assistance of submarine torpedoes in the channel and of several armed vessels and rams which together made the attempt of an enemy's fleet to attack or run by very hazardous. Nevertheless Flag - Oflicer Farragut, with the consent of his captains, prepared to try it.

The fleet, led by the Hartford, stole up the river in midnight darkness and quiet, and were not discovered until opposite the forts. Following the flagship, came the Richmond, her guns blazing and then came the Monongahela, the Kineo and the Mississippi — the last with George Dewey as executive officer, under Melancton Smith as commander. A furious battle ensued and the battle of Port Hudson has been pronounced by officers and seamen who were engaged in it, and who were present at the passage of Fort St. Philip and Fort Jackson, as the severest in the naval history of the civil war.

The Hartford got past and sailed on, but an accident to her machinery compelled the Richmond to try to turn around and escape before it was too late. She did so successfully, but when at the center of the semicircle of batteries, the Mississippi, close behind her, ran aground, and there was concentrated upon her the whole of the enemy's fire. This continued for half an hour, riddling her hull, ruining her upper works and smashing her machinery. All this time the vessel was replying with such vigor that more than two hundred and fifty shots were sent ashore in spite of the damage to the vessel. Then Captain Smith, seeing that there could be no hope of saving the ship, ordered an evacuation. The boats were then manned, the wounded were transported to the Union gunboat Genessee, which had approached to render assistance; the men were mostly landed in safety on the west bank, and a journey was made to and from the Richmond to place wounded men and officers on that vessel.

All of this time, the fire of the batteries continued, and Captain Smith and Lieutenant Dewey stayed on board and directed operations. A man was next sent to set fire to the fore storeroom, and did so; but before his blaze got well started, three of the enemy's cannon balls came through that part of the ship and let in enough water to drown the flames. Then other fires were started elsewhere in the cabins and hull, and the last boatload waited to see that they got well going, to prevent Confederate capture of a good ship. Then both officers escaped by boat to the Richmond, a mile below. Lightened in weight by the fire and by the removal of some three hundred men, the ship lifted from the mud and floated down the river, firing her still shotted guns and exploding one by one the shells that lay upon her decks, until she became dangerous to the Richmond and other Federal vessels near which she drifted.

Dewey was highly complimented, not only by his immediate superiors, but by Farragut himself, who now appointed him executive officer of the Agawam — a small gunboat, which the admiral made frequent use of as a dispatch boat, and for his personal reconnoitering. This little vessel was frequently fired at, by concealed sharpshooters or temporary batteries. In July of that year these attacks brought about a small engagement at Donaldsonville, Louisiana, in which Captain Abner Read, commander of the Monongahela, was killed and his executive officer severely wounded. Dewey was present, and was so conspicuous for gallantry that he was recommended for promotion on the strength of it; and meanwhile he was given temporary command of the frigate.

In the latter part of 1864, after some service in the James River under Commander McComb, Lieutenant Dewey was made executive officer of the first rate wooden man-of-war Colorado, which was stationed on the North Atlantic blockading squadron under command of Commodore Henry Knox Thatcher.

The blockade was an exceedingly important part of the plan of the war. The blockade was never made so perfect that no vessels could pass through, but it became nearly so toward the close of the war, and this was a matter of international importance as well as belligerent value in stopping the Confederates from receiving the foreign supplies upon which they so largely depended.

Large numbers of blockade runners were captured or driven ashore and wrecked. The profit on a single cargo that passed either way in safety was very great, and special vessels for blockade running were built in England. The Confederate government enacted a law providing that a certain portion of every cargo thus brought into its ports must consist of arms or ammunition, otherwise vessel and all would be confiscated. This ensured a constant supply. Clothing and equipments, too, for the Confederate armies came from the same source. To pay for these things, the Confederates sent out cotton, tobacco, rice, and the naval stores produced by the North Carolina forests. Strenuous efforts were constantly made to shut off this trade and communication, which made the traders of Great Britain and other European nations practically allies of the confederacy, and such officers as Lieutenant George Dewey had shown himself to be were needed, especially in the North Atlantic division, which covered such ports as Wilmington, where blockade running flourished.

It was to close the port of Wilmington, as much as to reduce the only coast fortification left to the South, that a powerful expedition, in which the navy was to co-operate with the army, was organized against Fort Fisher, at the mouth of Cape Fear River, in the early winter of 1864 to 1865. An attack delivered at Christmas proved a failure, and the land forces were largely withdrawn for service elsewhere. This conflict is known as the First Battle of Fort Fisher (December 7 – 27, 1864). It was followed by the Second Battle of Fort Fisher (January 13 – 15, 1865).

The navy remained and in the middle of January made a second attack, assisted by some soldiers under Terry, who were reinforced by marines and sailors from the ships. This was one of the hardest fought engagements on land and sea of the civil war and it resulted in a Federal victory, in which the navy, afloat and ashore, carried off the principal honors. The Colorado, being a wooden ship, was placed in the line outside the monitors and other armored vessels but got a full share of conflict. Toward the end of the second engagement, when matters were moving the right way, Admiral Porter signaled Thatcher to close in and silence a certain part of the works. As the ship had already received considerable damage, her officers remonstrated. But Dewey, who, had now acquired marked tactical ability, was quick to see the advantage to be gained by the move and the work was taken in fifteen minutes. The New York Times, commenting upon this part of the action, spoke of it as 'the most beautiful duel of the war.' When Admiral Porter came to congratulate Commodore Thatcher the latter said generously : 'You must thank Lieutenant Dewey, sir. It was his move.' Nevertheless Thatcher was promoted to be a rear - admiral and tried to take Dewey with him as his fleet captain when he went to supersede Farragut at Mobile Bay. This was not permitted, but Dewey was promoted to lieutenant - commander.

After the end of the civil war Lieutenant - Commander Dewey remained in active service, and was sent to the European station as executive officer of the Kearsarge — the famous old ship that had sunk the privateer Alabama.

Dewey's next tour of duty was in 1867 and 1868 as executive officer of the Colorado — the same vessel in which he had won his honors at Fort Fisher, and now the flagship of the European Squadron. The admiral in command of the ship and squadron was Goldsborough, and one of Dewey's companions was John Crittenden Watson — the same man, who, as rear - admiral, relieved Admiral Dewey of his duties at Manila, when he wished to return to the United States in the summer of 1899."

Some tranquil years followed the end of Dewey's cruise in the Colorado. For two years, from 1868 to 1870, he was an instructor at the Naval Academy. The next year he did special surveying work in the steamer Narragansett, and in 1872 was given command of that vessel, and spent nearly four years in her, engaged in the service of the Pacific Coast Survey."

This entitled him to a period of rest ashore; and he was ordered to Washington, and made lighthouse inspector in 1880, and subsequently secretary of the lighthouse board, a service in which he took great interest. Meanwhile he had been promoted to the grade of commander. This residence in Washington as a bureau officer of high rank gave him an extensive acquaintance, and he became one of the most popular men in the capital. He was a member of the Metropolitan Club, the leading social club of Washington.

In 1882, this leave of absence in Washington came to an end by his being sent to the Asiatic station in command of the Juniata, where he studied the situation with care and acquired information of immense importance ten years later.

The rank of captain was reached in 1884, and he was ordered home and given command of the Dolphin — one of the first four of the original white squadron, which formed the basis of the new and modern navy of the United States. The Dolphin was intended as a dispatch boat, and was often used as the president's yacht.

In 1885, Captain Dewey undertook another tour of sea service, and for three years was in command of the Pensacola, familiar to him in the New Orleans battles, now flagship of the European squadron."

Returning to Washington in 1893 he resumed the life of a bureau officer, being attached to the lighthouse board, and remained there until 1896. when he was commissioned commodore, and transferred to the board of inspection and survey.

Dewey felt, in 1897, that his health was suffering in the climate and inaction of Washington, and applied for sea duty. It was granted to him, and he was assigned to the command of the Asiatic station. He felt certain, as did so many others at Washington that year, that war with Spain was imminent although few had thought of the Philippines as a field of serious war.

The Commodore hoisted his pennant at Hong Kong in December, 1897, and immediately began preparations for wartime service. As early as January 1898 the Navy Department began to send him instructions, as it was doing to other commanders under the administration of Secretary of Navy John D. Long and Assistant Secretary of Navy Theodore Roosevelt. Dewey was ordered in January to retain all enlisted men whose terms had expired; and a month later was told to keep the Olympia, instead of sending her back to San Francisco. He was instructed to assemble all his squadron at Hong Kong, and to fill all the bunkers with coal. At the same time the cruiser Baltimore was dispatched to him from the United States, via Hawaii; and at Honolulu was met by the steamer Mohican from San Francisco, which transferred to her a shipload of ammunition, sent far in advance of its possible use.

Dewey's ships were scattered up and down the Asiatic coast; but by the end of March the whole squadron, except the antiquated wooden Monocacy, had been gathered in the port of Hong Kong, their coal and stores replenished. Then came a period of waiting, the commodore was constantly making ready. First he sent the fleet paymaster over to the consignees of the English steamship Nanshan, and bought her as she was, with 3,300 tons of coal on board. Then he bought the Zafiro, a steamship of the Manila - Hong Kong line, just as she was, with all her fuel and provisions, and on her was placed all the spare ammunition, so that she became the magazine of the fleet.

On April 18, the McCulloch came in and joined the squadron. She was a revenue cutter but she was as good as a gunboat, being built of steel, having 1,500 tons displacement, and carrying four 4-inch guns and a crew of one hundred and thirty men. On the 21st, when General Woodford was leaving Madrid, and Señor Luís Polo de Bernabé was slipping out of Washington, the Baltimore appeared, a powerful addition to the fleet, and bringing also her load of ammunition, so that she was doubly welcome.

As the news now daily published in Hong Kong made war seem certain, all the white vessels were repainted war - gray, and the last possible preparations made when the cable brought word of the declaration of war, to date from April 22 and also of England's declaration of neutrality. Word was therefore sent to the American commander by the Governor of Hong Kong that his vessels could no longer be harbored there. That was no hardship, for they were as completely outfitted as they cared to be, and only a few miles away were the Chinese waters of Mirs Bay, where nobody would or could interfere with their anchorage. Thither Dewey took his ships on April 25, leaving the McCulloch to bring last dispatches; and the next day she joined the fleet in a hurry, taking to the commander the following fateful message from the Government of the United States:

'Dewey, Asiatic Squadron: "War has commenced between the United States and Spain. Proceed at once to Philippine Islands. Commence operations at once, particularly against the Spanish fleet. You must capture vessels or destroy. Use utmost endeavors. Long."

This was on the 26th. At 2:00 p.m. the next day, April 27, Dewey's squadron was leaving Mirs Bay for the Philippine Islands, in search of another squadron of warships as large and as new and as well armed as itself, to seek the first naval encounter of modern ships and with modern ordnance.

On April 27, 1898, he sailed out from China with orders to attack the Spanish at Manila Bay. He stopped at the mouth of the bay late the night of April 30, and the following morning he gave the order to attack at first light, by saying the now famous words "You may fire when you are ready, Gridley." Within 6 hours, on May 1, he had sunk or captured the entire Spanish Pacific fleet under Admiral Patricio Montojo y Pasarón and silenced the shore batteries at Manila, with the loss of only one life on the American side. Dewey aided General Wesley Merritt in taking formal possession of Manila on August 13, 1898. In the early stages of the war the Americans were greatly aided by the Filipino nationalists led by Emilio Aguinaldo who had been attacking the Spanish by land as Dewey was attacking them by sea. Dewey and Aguinaldo at first enjoyed a cordial relationship, and Dewey wrote that the Filipinos were “intelligent” and well "capable of self - government"; however the McKinley administration soon decided otherwise, and by the start of 1899, Dewey had to threaten to shell Aguinaldo's forces to allow American troops to land in Manila.

The totality of Dewey's victory brought the world to a sharp awareness of the United States as a naval power. Returning to the United States in 1899, he received a hero's welcome. New York City's September 1899 welcome home celebration for Dewey was a two day parade served more to promote the potential of American colonialism in Asia than to honor Dewey's heroism in the Philippines. Newspapers transformed Dewey into the epitome of American superiority by highlighting his leadership capabilities and accentuating the strength and masculinity of his features. Dewey's swift easy victory encouraged the William McKinley administration in its decision to place the Philippines under American control.

Dewey returned to America to a hero's welcome, and by act of Congress was made Admiral of the Navy in 1899. A special military decoration, the Battle of Manila Bay Medal (commonly called the Dewey Medal), was struck in his honor of his achievement at Manila Bay and awarded to every officer, sailor and Marine present at the battle.

The U.S. Congress awarded the title of Admiral of the Navy to Dewey in 1903. In 1900 he was named president of the newly established General Board of the Navy Department, which set basic policy. He served in this post until his death in Washington, January 16, 1917. He is interred in the Bethlehem Chapel, on the crypt level, at the Washington National Cathedral, Washington, DC.

Dewey was energetic, competent and vain. He wore spiffy clothes and a glorious handlebar mustache; thanks to inherited wealth he lived in style. He often went horseback riding with Roosevelt in Washington's Rock Creek Park and he was a fellow member of Washington's prestigious Metropolitan Club.

After Dewey's return from the Spanish - American War, many suggested he run for President on the Democratic ticket in 1900. However, his candidacy was plagued by public relations missteps. Newspapers started attacking him as naïve after he was quoted as saying the job of president would be easy since the chief executive was merely following orders in executing the laws enacted by Congress and that he would "execute the laws of Congress as faithfully as I have always executed the orders of my superiors." Shortly thereafter he admitted to never having voted in a presidential election. He drew yet more criticism when he offhandedly told a newspaper reporter that "Our next war will be with Germany."

Dewey also angered some Protestants by marrying Catholic Mildred McLean Hazen (the widow of General William Babcock Hazen and daughter of Washington McLean, the owner of The Washington Post) in November 1899 and giving her the house that the nation had given him following the war.

Dewey withdrew from the race in mid-May 1900 and endorsed William McKinley.

Admiral Dewey's final rank was Admiral of the Navy which is considered a Six - Star Admiral. He was the only person ever to hold this rank. Admiral of the Navy is equal to General of the Armies, also considered to be a Six - Star Rank. General Pershing and General Washington held this rank, Washington long after his death.