US Army vs federal agency

US Army and federal agency both are nouns.

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

As nouns, federal agency is a hypernym of US Army; that is, federal 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
federal agency: an administrative unit of government

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

US Army (noun)	federal 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