Well, the Roman empire was eventually kicked in by force from the outside ... but it was a combination of Huns, Goths, and Vandals that finally got them (plus constant pressure from Persia, which more or less prompted the division of the Empire into Rome and the Byzantine Empire so that somebody would still be in charge of dealing with the Persians). Could such a military takeover have happened earlier, had an endless string of attackers hit them? I doubt the records exist to make a case for that, one way or the other.
But realistically, it is unlikely that empires who actively hated each other could have ever agreed to attack anybody simultaneously. The Germanic invasions were not organized - they were just nomads moving west because there were even worse people behind them (the Tartars and the Mongols), who were pushed into conflict with Rome just because Rome was there. It is doubtful that they would have just agreed to start beating on Rome, had they actually owned a place to call home. Likewise, in the early Roman Republic, for example, Gaul would have no reason to support the Macedonians or Carthage ... because even if they got together and beat Rome, then the Gauls would just have to fight Macedonia or Carthage. That's just not the way politics works.
Another thing that bugs me about computer strategy games ... you seldom see a message like "your people are tired of fighting and are going to start refusing to join the army unless they start seeing some benefits from this war." Where, realistically, that's a pretty common situation. Most real people don't think about political maneuvering with the objective of taking over the world ... their idea of good foreign policy is one where they personally don't have to go and fight (or if they do, it's for a darn good reason).
