What if the Mongols reached the New World?
In a parallel universe, abnormal weather in Asia leads to the formation of a second glacial land bridge between Asia and North America during the 1200s, while Genghis Khan is in power.
During the early stages of the Mongol conquests, Genghis Khan (for some reason) decides to go East instead of West, leading him to discover the land bridge and cross it into North America.
Congratulations, Genghis Khan! You just discovered the New World.
How screwed are the indigenous people of the New World now that Genghis Khan's Mongol Army is sweeping through their territory?