AHC: History's most dramatic conquests interrupted by a totally different faction

What if one of history's most dramatic conquests in terms of replacing institutions and cultures, was interrupted, not just by a leader losing a battle, but by an entirely different faction entering the conflict and taking the conquering power by surprise?

The closest "almost happened" example I can think of would be Khosrau II's invasion of the Eastern Roman Empire just before the early Islamic expansions. If Khosrau II won, then the newly expanded Persian Empire would have immediately been supplanted by the Caliphate.

ATL examples (not saying these are plausible):
  • If the Mongol Empire at its height had been usurped by Jurchens, leaving isolated Mongol forces fighting in Persia and the Pontic Steppe.
  • If Alexander the Great in the 320s faced a massive Gallic or Thracian invasion that threatened to usurp Greece
  • If the Umayyad Caliphate in the 690s was shattered by an invasion of Bulghars/Khazars
  • American Indian Wars delayed, if the War of 1812 led to the British re-annexing the United States, and a new Proclamation Line
  • Roman conquests interrupted, if Rome collapsed during what was in OTL the Pax Romana

The challenge is to pick one of the biggest conquests in history in terms of how it changed the region affected permanently. The more momentum seemed to have been behind it (such as the Mongol conquests) the better. Then, with this historical trend and/or specific war interrupted -- how would the OTL ruling caste or remnants of the conquest, deal with being isolated by the ATL surprise attack?
 
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One TL I'd like to do if I ever have time is where Britain c. 1890 or so falls to a revolution of some sort. I'm not sure how the colonies and dominions would react -- declare independence? accept the new regime? declare loyalty to Queen Victoria and fight to restore her to the throne? -- but it would be very interesting to see.
 
Another good POD could be the Cimbri and Teutones invading Italy immediately after the Battle of Orange. Given the poor performance of Roman arms against them previously, and the fact that Rome wouldn't have the time it had IOTL to get its act together, it's quite possible they'd be able to sack Rome itself.

Or, a bit later, have the allies successfully gain independence during the Social War. Rome still controls large parts of the Mediterranean, but their control in Italy has just collapses and they've now got a menacing confederation of cities right on their doorstep.
 
Another good POD could be the Cimbri and Teutones invading Italy immediately after the Battle of Orange. Given the poor performance of Roman arms against them previously, and the fact that Rome wouldn't have the time it had IOTL to get its act together, it's quite possible they'd be able to sack Rome itself.

Or, a bit later, have the allies successfully gain independence during the Social War. Rome still controls large parts of the Mediterranean, but their control in Italy has just collapses and they've now got a menacing confederation of cities right on their doorstep.
How Mithridatic Pontus develops in this scenario would be interesting.
 
Or, a bit later, have the allies successfully gain independence during the Social War. Rome still controls large parts of the Mediterranean, but their control in Italy has just collapses and they've now got a menacing confederation of cities right on their doorstep.
Maybe Roman Hispania could recede and become like an "al-Andalus" against the Celtiberians in the north.
 
I think a scenario that definitely fits the bill is Sweyn II of Denmark, who could have intervened during/after the Norman conquest and install Edgar Ætheling on the throne. Whether he would have uprooted the entire Norman aristocracy in England at that point is debatable, but history would have ended up much different for England.
 

jocay

Banned
While the Spanish are laying siege to Tenochtitlan, the Aztecs' historic enemy the Tarascan Empire decides to take advantage of the situation laid by Cortes and conquer all of Mexico for itself. As far as we know, the Tarascans would've provided a much more unitary, centralized administration to Mexico than the Aztec Triple Alliance. At least until the next wave of Europeans come to Mexico's shores.
 
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Delaying the end of the Sengoku Jidai in Japan while keeping the Ming still on the decline (the Imjin War was a costly endeavor for the Ming with very little payout and, more importantly, kept them distracted from Nurhaci's actions for a while, but wasn't the only factor between 1590 and 1618 that led to the Seven Grievances being announced and the ensuing decades of the Ming getting thrashed by the Jurchen/Manchu) and the Manchu could end up sandwiched between Japanese and Ming forces if/when the Japanese try their luck on the mainland (the Japanese attacked Korea because the Joseon king wouldn't let them pass through the peninsula to attack the Ming Empire and conquer China since that would be an act of betrayal and invite a later Ming invasion. If the Manchu are in the process of conquering China, the Joseon might not have such qualms about letting the Japanese march through since they'd be fighting the Manchu, whom most of the Joseon court had little love for).
 
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