Aggressor
Part of speech: noun
Definition: The person who first attacks or makes an aggression; he who begins hostility or a quarrel; an assailant.
Example sentence: What the American people want to do is fight a war without getting hurt. You can't do that any more than you can get into a barroom fight without getting hurt... Unless the American people are willing to send their sons out to fight an aggressor, there just isn't going to be any United States.
Assaulter
Part of speech: noun
Definition: One who commits assault.