The United States Navy

The mission of the Navy is to maintain, train and equip combat-ready Naval forces capable of winning wars, deterring aggression and maintaining freedom of the seas.—Mission statement of 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: