Union vs US

Union and US both are nouns.

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

Union (noun)	US (noun)
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

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