Attacking a region can be committed by a state president for 10 gold + 10% of active population (in gold).
To be able to attack a region, the following conditions have to be fulfilled:
-There must be an active war between two countries.
-There must not be an active battle at the attacked region.
-Attacker must have war initiative.
-Attacker must have enough gold to pay for the attack.
-There must be a border between attacker and the region that is being attacked.
You can only attack one region at a time per enemy.
Note that country can be attacked simultaneously by many enemies, as well as a country can launch attacks on multiple enemies.