If you have a hypothetical question, there's no better folks to ask than the team behind Kurzgesagt. What would happen if an alien civilization went to war with earth? Considering the vast distances between the two warring factions, the aliens would prefer to just destroy the entire earth with one shot, and we'd never know what hit us. Based on the technology we already know about, they might select from three weapons: the star laser, the relatavistic missile, and the ultra-relativistic electron beam.
Each one of these super-weapons is terrifying, even from light-years away. If you feel frightened by them, just remember that we'll never know what hit us. Still, all this is very unlikely because of the distances involved. What they're trying to get across to us is that war between planets is way more trouble than it's worth, no matter how angry they may be at us. Now imagine how much this science-based speculation would affect our science fiction stories and Marvel movies. This videois only eleven minutes long; the rest is an ad.