diversion vs onset

diversion and onset both are nouns.

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

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

diversion: an attack calculated to draw enemy defense away from the point of the principal attack
onset: (military) an offensive against an enemy (using weapons)

Other hypernyms of diversion include attack, onrush, onslaught.

diversion (noun)	onset (noun)
an activity that diverts or amuses or stimulates	(military) an offensive against an enemy (using weapons)
a turning aside (of your course or attention or concern)	the beginning or early stages
an attack calculated to draw enemy defense away from the point of the principal attack