Aggressor (noun)

1. A person or group that initiates an attack or invasion.
2. A person who is actively hostile or assaultive.

Origin:

From latin aggressor, from aggredi ("to attack").

Examples:

1. In the war, one country was the aggressor and the other country was the defender.
2. He was arrested for assault after attacking another person without provocation, making him the aggressor.
3. The aggressor's army was well-trained and well-equipped.
4. The police identified him as the aggressor in the altercation.
5. The aggressor nation was held responsible for starting the conflict.