From the Coast Guard web site. Click on the link if you want to learn more.

The U.S. Coast Guard is a military, multi-mission, maritime service. Operating within the Department of Transportation during peacetime, the Service falls under the direction of the Secretary of the Navy upon declaration of war or when the President directs. The Coast Guard is a unique federal agency. In addition to its non-redundant yet compelling national defense role as one of the five U.S. Armed Services, the Coast Guard is charged with a broad scope of regulatory, law-enforcement, humanitarian, and emergency-response duties. The Coast Guard operates in a complex and dangerous maritime environment characterized by rapidly changing security threats at home and abroad.