bridgehead vs combat zone

bridgehead and combat zone both are nouns.

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

As nouns, combat zone is a hypernym of bridgehead; that is, combat zone is a word with a broader meaning than bridgehead:

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

Other hypernyms of bridgehead include combat area.

bridgehead (noun)	combat zone (noun)
an area in hostile territory that has been captured and is held awaiting further troops and supplies	a military area where combat forces operate
a defensive post at the end of a bridge nearest to the enemy	a city district known for its vice and high crime rate