combat vs hostilities

combat and hostilities both are nouns.

Word	Noun	Adjective	Verb	Adverb
combat	Yes	No	Yes	No
hostilities	Yes	No	No	No

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

Other hyponyms of combat include belligerency, trench warfare, aggression.

combat (noun)	hostilities (noun)
an engagement fought between two military forces	fighting; acts of overt warfare
the act of fighting; any contest or struggle

combat (verb)	hostilities (verb)
battle or contend against in or as if in a battle