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I was thinking about how much of a problem usurpers were for the Roman Empire. Then I remebered that in 260 AD, during the Crisis of the Third Century, the Empire fractured into three parts, the Gallic Empire in Gaul and Britannia (and Hispania for a while I think) and the Palmyrene Empire in Syria, Mesopotamia and Egypt. The "real" Empire kept Italy, Africa, the Balkans and some of Asia.

Now, these successor states were soon reconquered, but they considered themselves to be separate states from the Roman Empire, not just provinces held by usurpers. From my limited knowledge of Roman history, this is unique. So what I'm asking is what conditions led to two states breaking away from the Empire at around the same time, rather than two usurpers trying to take the throne.
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