In a universe were everything is possible, the Roman Empire has survived, and come face to face with the Mongol Horde!
Note: Drawing not entirely accurate
Scenario:
In the 13th Century, the Mongol Horde slaughtered it's way through central and eastern Europe. With plans to control the Black Sea, the Mongol Empire attacks the Roman Empire in what is now Turkey and Romania. But who is victorious? That, is for you to decide. Feel free to edit the map if you like or discuss the short and long term effects of the war.
In my opinion, the Mongols would be victorious, but if they attacked further into Europe, then the Roman Empire would manage to defend themselves and defeat the invaders, possibly even pushing them back as far as the Caspian Sea.