(from uscg.mil)
The United States Coast Guard is a military, multimission, maritime service within the Department of Homeland Security and one of the nation's five armed services. Its core roles are to protect the public, the environment, and U.S. economic and security interests in any maritime region in which those interests may be at risk, including international waters and America's coasts, ports, and inland waterways.
The Coast Guard provides unique benefits to the nation because of its distinctive blend of military, humanitarian, and civilian law-enforcement capabilities. To serve the public, the Coast Guard has five fundamental roles:
Maritime Safety: Eliminate deaths, injuries, and property damage associated with maritime transportation, fishing, and recreational boating. The Coast Guard's motto is Semper Paratus—(Always Ready), and the service is always ready to respond to calls for help at sea.
Maritime Security: Protect America's maritime borders from all intrusions by: (a) halting the flow of illegal drugs, aliens, and contraband into the United States through maritime routes; (b) preventing illegal fishing; and (c) suppressing violations of federal law in the maritime arena.
Maritime Mobility: Facilitate maritime commerce and eliminate interruptions and impediments to the efficient and economical movement of goods and people, while maximizing recreational access to and enjoyment of the water.
National Defense: Defend the nation as one of the five U.S. armed services. Enhance regional stability in support of the National Security Strategy, utilizing the Coast Guard’s unique and relevant maritime capabilities.
Protection of Natural Resources: Eliminate environmental damage and the degradation of natural resources associated with maritime transportation, fishing, and recreational boating.