Defense Department vs US Army

Defense Department and US Army both are nouns.

Word	Noun	Adjective	Verb	Adverb
Defense Department	Yes	No	No	No
US Army	Yes	No	No	No

Defense Department (noun)	US 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