onslaught vs military operation

onslaught and military operation both are nouns.

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

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

onslaught: (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 onslaught include operation.

onslaught (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 rapid and continuous delivery of linguistic communication (spoken or written)
a sudden and severe onset of trouble