US Army vs government agency

US Army and government agency both are nouns.

Word	Noun	Adjective	Verb	Adverb
US Army	Yes	No	No	No
government agency	Yes	No	No	No

As nouns, government agency is a hypernym of US Army; that is, government agency is a word with a broader meaning than US Army:

US Army: the army of the United States of America; the agency that organizes and trains soldiers for land warfare
government agency: an administrative unit of government

Other hypernyms of US Army include agency, authority, bureau, federal agency, office.

US Army (noun)	government agency (noun)
the army of the United States of America; the agency that organizes and trains soldiers for land warfare	an administrative unit of government