"U.S. naval forces give the United States the ability to convert the world's oceans--a global commons that covers more than two-thirds of the planet's surface--into a medium of maneuver and operations for projecting U.S. power ashore and otherwise defending U.S. interests around the world. The ability to use the world's oceans in this manner--and to deny other countries the use of the world's oceans for taking actions against U.S. interests--constitutes an immense asymmetric advantage for the United States. As discussed elsewhere (see 'Defense Primer: Geography, Strategy, and U.S. Force Design), the size and composition of U.S. naval forces reflect the position of the United States as a Western Hemisphere power with a goal of preventing the emergence of regional hegemons in Eurasia. As a result, the U.S. Navy includes significant numbers of aircraft carriers, nuclear-powered attack submarines, large surface combatants, large amphibious ships, and underway replenishment ships."
CRS In Focus, IF10486
Federation of American Scientists: http://www.fas.org/sgp/crs/index.html