aggressor

Short definition from Wiktionary

Noun

1. 1
   The person or country that first attacks or makes an aggression; that begins hostility or a quarrel; an assailant.
2. 2
   a citizen/subject of the fictional Aggressor state

Proper noun

1. 1
   a fictional state created by the United States Army for maneuver and military exercise purposes in the 1950's