United States Army vs agency

United States Army and agency both are nouns.

Word	Noun	Adjective	Verb	Adverb
United States Army	Yes	No	No	No
agency	Yes	No	No	No

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

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

Other hypernyms of United States Army include authority, bureau, federal agency, government agency, office.

United States Army (noun)	agency (noun)
the army of the United States of America; the agency that organizes and trains soldiers for land warfare	to have the ability to produce a particular effect or achieve an end
	an administrative unit of government
	a business or organization that provides a particular service, especially the mediation of transactions between two parties
	the state of serving as an official and authorized delegate or agent
	the state of being in action or exerting power