In a bridge battle in russia, it seemed that the Bridge made my cursor go into an X. I couldn't make a unit move onto the bridge, over the bridge, attack a unit on the bridge, nothing. The enemy ran over the bridge, got killed, ran back halfway to the middle of the bridge, and just waited there. to get a unit to attack the others, I told them to just run to the other side of the bridge and then stop as soon as they were standing a foot from the enemy.

Once the unit was on the bridge, however, my cursor changed to an X and stayed that way-if I moved it to the river, to the bridge, to another unit, to the ground. I left-clicked to my general(who I hadn't moved the entire battle) and the cursor was still an X.