Challenge: Byzantine-Sassanid Alliance

In before someone says "Caliphate":

The Huns have more success and wreak havoc in Asia Minor and Syria during a period of relative anarchy (perhaps after a major Byzantine/Persian war) and this chaos leaves a huge power vacuum. Hunnic soldiers capture Ctesiphon and the Sassanian government retreats into the Iranian plateau, which the Huns have difficulty taking. These early victories encourage a flood of new migrants.

Over the next generation or so the Byzantines and Sassanians formally cooperate to bring down this new invader.
 
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