U.S. vs Union

U.S. and Union both are nouns.

Word	Noun	Adjective	Verb	Adverb
U.S.	Yes	No	No	No
Union	Yes	Yes	No	No

U.S. (noun)	Union (noun)
the executive and legislative and judicial branches of the federal government of the United States	the United States (especially the northern states during the American Civil War)
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

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