invasion (n.) Look up invasion at Dictionary.com
mid-15c., from Old French invasion "invasion, attack, assaut" (12c.), from Late Latin invasionem (nominative invasio) "an attack, invasion," noun of action from past participle stem of Latin invadere "go into, fall upon, attack, invade," from in- "in" (see in- (2)) + vadere "go, walk" (see vamoose).