A CO power is an ability which most COs in the Advance Wars series have. They can have a variety of effects, from causing massive damage, spawning units, or repairing. As such they can often change the tide of the battle. With each Advance Wars series, slight modifications have been made to CO powers, though the principal remains the same in all. Each time a player attacks with a Unit, or a Unit is attacked, they gain a small amount of Star power, which fills a bar. Once the bar is full then the players may use their COs' CO power.