This is easier said than done, as there are generally a lot of powerful enemy troops to stop you. Your units are, to put it kindly, less than effective on the attack and defense, so you as Sarge will end up doing nearly all of the work. Since it is so easy to die, and games cannot be saved while underway, you'll end up having to restart missions many times to complete them. Saving occurs only when you complete a mission within the campaign, and then happens automatically. This system is baffling in its stupidity.
Missions can be quite long and involve a lot of battles.
Having to perform perfectly in every one to complete the mission is asking a lot. It also means you can't play part of a mission and take a break. It's all or nothing. The campaign generally follows Sarge as he attempts to thwart the Tan Army. I won't give away the plot, but it's pretty interesting in its way.