Actually, they (enemy reinforcementts) are pretty much guaranteed to come from directly behind you. (Seems like CA figures every reinforcement should be a reenactment of Waterloo). I really, really wish that (a) we had limited control of reinforcements and (b) they came from directions that corresponded to the strategic map. In fact, I really don't understand why E:TW is doing reinforcements as they do instead of as M2TWK did (which was a huge step in the right direction).