You can increase your Action Points by freeing P.O.W.s. When a level starts, tap the P.O.W. icon at the bottom right to learn what P.O.W. unit you are rescuing. Use that to decide which unit to select for the mission. Each area has four stages that have certain conditions for rescuing a P.O.W.s. You will need to repeat stages with different unit combinations to complete it quickly in order to rescue the P.O.W.s. Rescuing the second P.O.W. in a stage will be more difficult, as the game knows the limits of your unit allocation. You will need to use a combination of your available units to rescue the second P.O.W. Replay levels until all prisoners are freed to earn various bonuses.