Union vs USA

Union and USA both are nouns.

Word	Noun	Adjective	Verb	Adverb
Union	Yes	Yes	No	No
USA	Yes	No	No	No

Union (noun)	USA (noun)
the United States (especially the northern states during the American Civil War)	the army of the United States of America; the agency that organizes and trains soldiers for land warfare
	North American republic containing 50 states - 48 conterminous states in North America plus Alaska in northwest North America and the Hawaiian Islands in the Pacific Ocean; achieved independence in 1776

Union (adjective)	USA (adjective)
being of or having to do with the northern United States and those loyal to the Union during the American Civil War