onset vs military operation

onset and military operation both are nouns.

Word	Noun	Adjective	Verb	Adverb
onset	Yes	No	No	No
military operation	Yes	No	No	No

As nouns, military operation is a hypernym of onset; that is, military operation is a word with a broader meaning than onset:

onset: (military) an offensive against an enemy (using weapons)
military operation: activity by a military or naval force (as a maneuver or campaign)

Other hypernyms of onset include operation.

onset (noun)	military operation (noun)
(military) an offensive against an enemy (using weapons)	activity by a military or naval force (as a maneuver or campaign)
the beginning or early stages