countermove vs onset

countermove and onset both are nouns.

Word	Noun	Adjective	Verb	Adverb
countermove	Yes	No	No	No
onset	Yes	No	No	No

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

countermove: an attack by a defending force against an attacking enemy force in order to regain lost ground or cut off enemy advance units etc.
onset: (military) an offensive against an enemy (using weapons)

Other hypernyms of countermove include attack, onrush, onslaught.

countermove (noun)	onset (noun)
an attack by a defending force against an attacking enemy force in order to regain lost ground or cut off enemy advance units etc.	(military) an offensive against an enemy (using weapons)
	the beginning or early stages