The merchant marine is not part of the military, the term refers to a commercial industry, I assume that people who think merchant mariners are in the military also think Alaska Airlines i part of the Air Force.
Now to answer your question, it depends on how you define combat, many in the USN are in combat, yet never hear shot fired, same with the USAF, for that matter there are many in the Army or Marines who, while technically in combat are never really near the "front" (though in most modern wars the "front" wasn't real specific)
A cook on a navy destroyer is more likely to be in combat than an instructor in the Army
The Legislative branch
"In the United States, the power to declare war and appropriate military funds lies with the legislative branch of government, but the executive branch supplies the commander in chief of the military, capable of some military actions even without an official declaration of war." Quote from Wikipedia.
"In the United States, the power to declare war and appropriate military funds lies with the legislative branch of government, but the executive branch supplies the commander in chief of the military, capable of some military actions even without an official declaration of war." Quote from Wikipedia.
The abbreviation for the sea military branch is USN, which stands for United States Navy. Also, the United States Coast Guard (USCG) is a sea military branch in times of war.
The army is a branch of the military so really it was both of them.
US Army
Most likely there will be at least one.
Air Force
The Air Force .
Airforce.
US Navy.
WW1 , WW2 and the Korean conflict .