12-17-2011, 03:37 PM
I think Robert has hit the nail on the head in respect to this. The civil war with Magnentius seems to have pulled most of the army from Gaul, hence the need for serious military action led by a "royal" figure in the end to re-establish the frontiers - indeed the campaign that ended with Julian's victory over the Alamanni involved multiple armies and Constantius reasonably close at hand.