combat zone vs foothold

combat zone and foothold both are nouns.

Word	Noun	Adjective	Verb	Adverb
combat zone	Yes	No	No	No
foothold	Yes	No	No	No

As nouns, foothold is a hyponym of combat zone; that is, foothold is a word with a more specific, narrower meaning than combat zone:

combat zone: a military area where combat forces operate
foothold: an area in hostile territory that has been captured and is held awaiting further troops and supplies

Other hyponyms of combat zone include war zone, bridgehead.

combat zone (noun)	foothold (noun)
a military area where combat forces operate	an initial accomplishment that opens the way for further developments
a city district known for its vice and high crime rate	a place providing support for the foot in standing or climbing
	an area in hostile territory that has been captured and is held awaiting further troops and supplies