Wargame scenarios can be many things, they don't always have to recreate the Invasion of Normandy or some other large actual event, sometimes a good wargame can be a simple incident.
The bridge will create a choke point where the Germans will have to defend while the supply wagons get a reasonable distance away. They can only retreat when the wagons are far enough away that the Russians won't be able to catch them. The intent of the game is not that the Germans have to annihilate the Russians or even that the Russians have to destroy all the Germans. As long as the Germans can delay the Russians so they can get the wagons away, then they win. The Russian player has to move quickly or the wagons will escape. Good hunting to both!