A navy (maritime force) is the branch of a nation's armed forces principally designated for naval warfare and amphibious warfare; namely, lake- or ocean-borne combat operations and related functions. It includes anything conducted by surface ships, amphibious ships, submarines, and seaborne aviation, as well as ancillary support, communications, training, and other fields; recent developments have included space related operations. The strategic offensive role of a navy is projection of force into areas beyond a country's shores (for example, to protect sea-lanes, ferry troops, or attack other navies, ports, or shore installations). The strategic defensive purpose of a navy is to frustrate seaborne projection-of-force by enemies. The strategic task of the navy also may incorporate nuclear deterrence by use of nuclear missiles.

Naval warfare developed when humans first fought from water-borne vessels. Prior to the introduction of the cannon and ships with sufficient capacity to carry the large guns, navy warfare primarily involved ramming and boarding actions. In the time of ancient Greece and the Roman Empire, naval warfare centered on long, narrow vessels powered by banks of oarsmen (such as triremes and quinqueremes) designed to ram and sink enemy vessels or come alongside the enemy vessel so its occupants could be attacked hand-to-hand. Naval warfare continued in this vein through the Middle Ages until the cannon became commonplace and capable of being reloaded quickly enough to be reused in the same battle. The Chola Dynasty of medieval India was known as a one of the greatest naval powers of its time in the Indian Ocean. In ancient China, large naval battles were known since the Qin Dynasty (also see Battle of Red Cliffs, 208), employing the war junk during the Han Dynasty. However, China's first official standing navy was not established until the Southern Song Dynasty in the 12th century, a time when gunpowder was a revolutionary new application to warfare.

The mass and deck space required to carry a large number of cannon made oar-based propulsion impossible and ships came to rely primarily on sails. Warships were designed to carry increasing numbers of cannon and naval tactics evolved to bring a ship's firepower to bear in a broadside, with ships-of-the-line arranged in a line of battle.

The development of large capacity, sail-powered ships carrying cannon led to a rapid expansion of European navies, especially the Spanish and Portuguese navies which dominated in the 16th and early 17th centuries, and helped propel the age of exploration and colonialism. The repulsion of the Spanish Armada (1588) by the English fleet revolutionized naval warfare by the success of a guns-only strategy and caused a major overhaul of the Spanish Navy, partly along English lines, which resulted in even greater dominance by the Spanish. From the beginning of the 17th century the Dutch cannibalized the Portuguese Empire in the East and, with the immense wealth gained, challenged Spanish hegemony at sea. From the 1620s, Dutch raiders seriously troubled Spanish shipping and, after a number of battles which went both ways, the Dutch Navy finally broke the long dominance of the Spanish Navy in the Battle of the Downs (1639).

England emerged as a major naval power in the mid-17th century in the first Anglo-Dutch war with a technical victory. Successive decisive Dutch victories in the second and third Anglo-Dutch Wars confirmed the Dutch mastery of the seas during the Dutch Golden Age, financed by the expansion of the Dutch Empire. The French Navy won some important victories near the end of the 17th century but a focus upon land forces led to the French Navy's relative neglect, which allowed the Royal Navy to emerge with an ever-growing advantage in size and quality, especially in tactics and experience, from 1695. Throughout the 18th century the Royal Navy gradually gained ascendancy over the French Navy, with victories in the War of Spanish Succession (1701–1714), inconclusive battles in the War of Austrian Succession (1740–1748), victories in the Seven Years' War (1754–1763), a partial reversal during the American War of Independence (1775–1783), and consolidation into uncontested supremacy during the 19th century from the Battle of Trafalgar in 1805. These conflicts saw the development and refinement of tactics which came to be called the line of battle.

The next stage in the evolution of naval warfare was the introduction of metal plating along the hull sides. The increased mass required steam-powered engines, resulting in an arms race between armor and weapon thickness and firepower. The first armored vessels, the French FS Gloire and British HMS Warrior, made wooden vessels obsolete. Another significant improvement came with the invention of the rotating turrets, which allowed the guns to be aimed independently of ship movement. The battle between the CSS Virginia and the USS Monitor during the American Civil War (1861–1865) is often cited as the beginning of this age of maritime conflict. The Russian Navy was considered the third strongest in the world on the eve of the Russo-Japanese War, which turned to be a catastrophe for the Russian military in general and the Russian Navy in particular. Although neither party lacked courage, the Russians were defeated by the Japanese in the Battle of Port Arthur, which was the first time in warfare that mines were used for offensive purposes. The warships of the Baltic Fleet sent to the Far East were lost in the Battle of Tsushima. A further step change in naval firepower occurred when the United Kingdom launched HMS Dreadnought (1906), but naval tactics still emphasized the line of battle.

The first practical military submarines were developed in the late 19th century and by the end of World War I had proven to be a powerful arm of naval warfare. During World War II, the German Navy's submarine fleet of U-boats almost starved the United Kingdom into submission and inflicted tremendous losses on U.S. coastal shipping. The German battleship Tirpitz, a sister ship of the Bismarck, was almost put out of action by miniature submarines known as X-Craft. The X-Craft severely damaged her and kept her in port for some months.

A major paradigm shift in naval warfare occurred with the introduction of the aircraft carrier. First at Taranto in 1940 and then at Pearl Harbor in 1941, the carrier demonstrated its ability to strike decisively at enemy ships out of sight and range of surface vessels. The Battle of Leyte Gulf (1944) was arguably the largest naval battle in history; it was also the last battle in which battleships played a significant role. By the end of World War II, the carrier had become the dominant force of naval warfare.

World War II also saw the United States become by far the largest Naval power in the world, with over 70% of the world's total numbers and total tonnage of naval vessels of 1,000 tons or greater. Throughout the rest of the 20th century, the United States Navy would maintain a tonnage greater than that of the next 17 largest navies combined. Of that 30% 19% of the tonnage comes from the Royal Navy. The Royal Navy is classed second in the world in terms of naval fire power and gross tonnage. The Royal Navy is also the only country outside of the USA to be building large Super Carriers. (A super carrier is one of the largest aircraft carriers in service with a gross tonnage of 70,000 tonnes or more.)