army vs ground forces

army and ground forces both are nouns.

Word	Noun	Adjective	Verb	Adverb
army	Yes	No	No	No
ground forces	Yes	No	No	No

As nouns, army and ground forces are synonyms defined as:

army and ground forces: a permanent organization of the military land forces of a nation or state

Other synonyms of army include regular army.

army (noun)	ground forces (noun)
a permanent organization of the military land forces of a nation or state	a permanent organization of the military land forces of a nation or state
a large number of people united for some specific purpose