Did you know that terms people use to describe the services collectively aren't interchangeable? For instance, "the military services" and "the armed forces" don’t mean the same thing.
There are four military services: The Army, Marine Corps, Navy and Air Force.
The military services are all part of the Defense Department.
Just three of the military services are departments: The Department of the Army, the Department of the Navy and the Department of the Air Force. The Marine Corps falls under the Department of the Navy.
There are five armed forces: The four military services plus the Coast Guard.
The Coast Guard is not administered by the DOD. Instead, it is part of the Department of Homeland Security.
During wartime, the president or Congress may direct that the Coast Guard operate as part of the Navy. During peacetime, the Coast Guard and the Navy often coordinate their activities.
There are seven uniformed services: The five armed forces plus the Public Health Service Commissioned Corps, which falls under the Department of Health and Human Services, and the National Oceanic and Atmospheric Administration Officer Corps, which falls under the Commerce Department.
Officers in the PHS Corps and NOAA Corps wear Navy uniforms and use Navy ranks. There are no enlisted or warrant personnel in these services.
The U.S. surgeon general, a vice admiral, directs the PHS Corps, which provides licensed medical and health sciences professionals to the PHS, DHHS, other uniformed services and other government agencies.
The NOAA Corps is made up of technically skilled science officers who can be incorporated into the armed forces in wartime. In peacetime, the corps supports defense requirements in addition to its purely nonmilitary scientific projects at sea, in the air and in laboratories.
The Merchant Marine is made up of civilian mariners and merchant vessels engaged in commerce or transportation of goods and services in and out of the navigable waters of the United States.
While the Merchant Marine isn’t a government agency or military service, in wartime it can become a Navy auxiliary service to deliver military personnel and materiel. The Navy also maintains a Department of Naval Science at the Merchant Marine Academy.
Merchant Marine Academy graduates are required to spend eight years as a commissioned officer in the reserve or National Guard component of any branch of the armed forces.