Have a mission where you are suppose to assasinate this officer in an enemy town, but it goes wrong so you call in for reinforcements and there is a large invasion that you join up with. However, before that happens, your troops get ambushed by some helicopters and tanks. You are severely outnumbered and have to retreat to the nearest friendly town. From there you quickly set up and defend the town against the attack. If the invaders take over, they will mass murder everyone in the town, including you!