surprise attack vs onset

surprise attack and onset both are nouns.

Word	Noun	Adjective	Verb	Adverb
surprise attack	Yes	No	No	No
onset	Yes	No	No	No

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

Other hypernyms of surprise attack include attack, onrush, onslaught.

surprise attack (noun)	onset (noun)
an attack without warning	(military) an offensive against an enemy (using weapons)
	the beginning or early stages