Two-player mode facilitates gameplay by adding two more units on the battlefield.
The goal is straightforward: destroy all enemy forces in each level.
You control a tank and a helicopter, both of which must work in tandem to succeed. Timing of shots is critical, as the tank has a certain level of inertia when moving, and you must shoot where the enemy tank will be when the shot lands, not where he presently is.
Needless to say, two-player mode is a lot more fun because you have the help of two more units on the battlefield. As you progress through each level, you will be promoted from Private when your scores are high enough, and awarded new units to command.