Why is the U.S. Marine Corps part of the Department of the Navy?

The United States Marine Corps is part of the Department of the Navy because it is specifically charged with conducting operations—including land and air missions—that are incident to naval campaigns.1

Historically, the status of the Marines was often uncertain, with duties split between army command on land and naval command at sea. This changed in 1834 with the passage of "An Act for the Better Organization of the United States Marine Corps," which formally established the corps as a permanent part of the naval hierarchy, while maintaining its identity as a distinct service.2

Today, the Marine Corps remains a separate military service within the Department of the Navy, specializing in amphibious landings, the defense of advanced bases, and providing security for naval installations and diplomatic missions. This organizational structure facilitates close cooperation with the navy, a partnership that has been essential for developing and executing modern amphibious warfare doctrines.13