theatre of operations vs combat zone

theatre of operations and combat zone both are nouns.

Word	Noun	Adjective	Verb	Adverb
theatre of operations	Yes	No	No	No
combat zone	Yes	No	No	No

theatre of operations (noun)	combat zone (noun)
a region in which active military operations are in progress	a military area where combat forces operate
	a city district known for its vice and high crime rate