I would say any empire or confederation is doomed when it starts fighting against itself (civil war) on a regular basis (not just a one-off). Repeated civil wars gravely weakened both the Seleucid and Ptolemaiac empires, so Rome could beat them later. Civil war destroyed the Roman Republic, Rome only survived by becoming an empire.

The Romans only beat the Gauls because they were constantly fighting each other, by the time they banded together against Rome, it was too late, too many warriors were already dead. Same goes for the Britons. Rome conquered through 'divide and rule' - persuading barbarian enemies to fight each other, fight on Rome's side against a rival tribe, or at least to stand aside and do nothing while Rome destroyed a rival tribe.