Although it is impossible to point to only one event I believe that the Gaulish tribes could have survived Caesar's expedition had they not asked from the Germans to interfer a few decades prior. Still they had been fighting each other for centuries. Imagine what the belgae alone could had done had they been united against Caesar. He says that they alone could muster 200,000 troops.
As for the Seleucids I think that it was a matter of time before their empire crubmled. The persian administrative managment (also used by Alexander) had serious problems maintaining the satrapes loyalty who either rebeled and formed their own kingdoms or sided with foreign ones.
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