Usdk wrote:
keep in mind that as soon as the empire stopped expanding, military service(ie generalship, cough cough caesar) was less appealing to the best and brightest, so they no longer entered in if they were not forced.
this weakend the military drastically.
having pissed off all the barbarians for a few generations, combined with them migrating west due to whatever conflicts were going on in the east.
That wasn't the case. Military service stopped being attractive because the Romans came to see it as a fools' errand.
They did not have anything comparable to the GI bill or VA. Being a legionary meant spending a VERY long period - as long as 20 years or longer - under the absolute command of a general who could take his army and march it wherever he pleased. In the case of men like Agricola or Caesar or Cato or Pompey this could mean finding yourself marching from Rome to Ireland or Judaea or Belgium or some other uncharted region, to fulfill one man's quest for glory. You would return home old and worn-out or not at all.
The problem wasn't that they didn't have leaders, it was that they didn't have soldiers, because of short-sighted policies and cynicism by previous generations, and enlistment was low as a result. A really compelling tale in this respect is Tacitus' account of the life of Germanicus - describing his relationship with his men and their complaints about the conditions of Roman military life.
What drove the Romans to invade Rome wasn't past grievances. The ancient world was not one in which neighboring countries were, all other things being equal, at peace: back then, the strong preyed on the weak and wars were endemic. Barbarians were driven to invade Rome because they themselves were pursued by other barbarians whose land was seized by...other barbarians who are not well known to us but lived roughly in what is today Belarus. This sort of "musical chairs" chains of invasions and migrations was a common theme in the ancient world.
