The United States Navy

From the New Recruits Handbook:
The mission of the United States Navy is to protect and defend the right of the United States and our allies to move freely on the oceans and to protect our country against her enemies.
The United States Navy serves as a seaborne branch of the Military of the United States. 10 U.S.C. § 5062 prescribes the Navy's three primary areas of responsibility:
"The preparation of naval forces necessary for the effective prosecution of war"