Union Army vs ground forces

Union Army and ground forces both are nouns.

Word	Noun	Adjective	Verb	Adverb
Union Army	Yes	No	No	No
ground forces	Yes	No	No	No

As nouns, ground forces is a hypernym of Union Army; that is, ground forces is a word with a broader meaning than Union Army:

Union Army: the northern army during the American Civil War
ground forces: a permanent organization of the military land forces of a nation or state

Other hypernyms of Union Army include army, regular army, blue.

Union Army (noun)	ground forces (noun)
the northern army during the American Civil War	a permanent organization of the military land forces of a nation or state