United States Marines vs US Marine Corps

United States Marines and US Marine Corps both are nouns.

Word	Noun	Adjective	Verb	Adverb
United States Marines	Yes	No	No	No
US Marine Corps	Yes	No	No	No

As nouns, United States Marines and US Marine Corps are synonyms defined as:

United States Marines and US Marine Corps: an amphibious division of the United States Navy

Other synonyms of United States Marines include Marine Corps, Marines, United States Marine Corps, USMC.

United States Marines (noun)	US Marine Corps (noun)
an amphibious division of the United States Navy	an amphibious division of the United States Navy