Well We have to consider them also to our advantage as well, at the time it would have seem as though we could have prevented them from crossing that point at which they were fighting, to save our arses. But Were going to need a way back over there, hmm if only we had some cargo planes and ships. Hmm...

Would destroying bridges well behind the front do anything? (Like cut off supply,reinforcement,etc)

Possibly, though there are several other routes stratigically, that one specific route is there for us to gain control of that area, while trying to get advance. However the campaign is "dynamic" and anything can happen. I am not sure of what I said being entirely accurate.