Navy, a nation's armed forces that operate on water. A navy consists not only of combat vessels, but also of aircraft and everything necessary to supply the fighting craft and maintain them in operation. Thus, naval bases, airports, and training centers are also a part of a nation's navy.

A navy's purpose is to defend its home shores against a conventional attack or invasion, to defend against ballistic-missile submarines, to protect its merchant shipping, and to provide support for invasions. To attain these objectives, a navy must be capable of gaining control of the sea, or that part of the sea essential to the nation's interests. When a navy controls the sea, its ships and its country's merchant vessels are free (or relatively free) from danger of attack by the enemy. Control of the sea also means that the navy can prevent supplies from reaching the enemy by sea.

Naval ships may be used to bombard shore installations and defenses. Such bombardment is routinely employed to soften up enemy defenses preceding amphibious (combined land-and-sea) operations. Only the largest navies have the capability to make major amphibious landings, which require landing vessels of many types for troops, weapons, vehicles, and equipment. Landing forces may be army troops or marines (naval personnel trained for land warfare and in the tactics of amphibious assault).

Each of the navies of the major powers— the United States, Russia, Great Britain, and France—also includes ballistic-missile submarines, which are a part of their strategic nuclear forces. These submarines can launch nuclear-armed missiles deep into enemy territory and inflict massive destruction. Consequently, these nations, while maintaining the ability to fight a conventional war, have developed the capacity to counteract the nuclear threat of the ballistic-missile submarines.