Department of Defense vs U. S. Army

Department of Defense and U. S. Army both are nouns.

Word	Noun	Adjective	Verb	Adverb
Department of Defense	Yes	No	No	No
U. S. Army	Yes	No	No	No

Department of Defense (noun)	U. S. Army (noun)
the federal department responsible for safeguarding national security of the United States; created in 1947	the army of the United States of America; the agency that organizes and trains soldiers for land warfare