United States Army vs authority

United States Army and authority both are nouns.

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

As nouns, authority is a hypernym of United States Army; that is, authority 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
authority: an administrative unit of government

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

United States Army (noun)	authority (noun)
the army of the United States of America; the agency that organizes and trains soldiers for land warfare	the power or right to give orders or make decisions
	official permission or approval
	freedom from doubt; belief in yourself and your abilities
	an authoritative written work
	an administrative unit of government
	an expert whose views are taken as definitive
	(usually plural) persons who exercise (administrative) control over others