When you play the game for the first time you'll select a side to fight for. You'll then be presented with a virtual map of Europe and North America, with front lines defined on it. Let's say that there's a fierce battle for Nebraska. You can choose to help out your side by jumping into a battle there. At the end of the day, the game calculates the overall winner based on the outcome of all the battles in that territory and adjusts the front lines accordingly. This cycle is repeated until one side is wiped out, and the war is reset.