From the Document: "Although the term 'naval forces' is often used to refer specifically to Navy forces, it more properly refers to both Navy and Marine Corps forces, because both the Navy and Marine Corps are naval services. For further discussion, see 'Defense Primer: Department of the Navy.' For a discussion of the Marine Corps that focuses on its operations as a ground-combat force, see 'Defense Primer: Ground Forces.' [...] 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 a regional hegemon in one part of Eurasia or another. 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