The take-home impression I got from reading through those two Cesar wars is that the legions were almost as dependent on local grain available to steal (ready to take in granary, or just ready to harvest, doesn't really matter) as trains are dependent on rails. I assume that except for bumping into an adequate rival in the east, they just gobbled up the entire "wheatosphere", quickly running out of steam (and, with a few notable exceptions, out of motivation) whatever they ran into hunter/gatherer economies.