The Mongol conquest of Europe may have been prevented by something as mundane as bad weather.
New research published Friday suggests the Mongol army, largely made up of cavalry (soldiers on horseback), couldn't deal with the harsh Hungarian winter of 1242.
Under the leadership of the fearsome Genghis Khan, the Mongols had conqured most of Asia and were making inroads into Europe when he died. His descendants picked up where he left off, pushing into Russia and eastern Europe, crossing the Danube River in 1242 into western Hungary.
But two months later Mongol troops withdrew, never to return.
"No reason is given in the Mongol sources to explain their abrupt departure from Hungary," the researchers write in journal Scientific Reports.
Previous explanations have included stiff resistance from Polish and Hungarian troops, and the death of Great Khan Ogedei -- the Mongol leader at the time. But the Mongols won every battle they fought in Europe, and a Mongol historian writing in the decades after said the forces were not aware of Ogedei's death when the withdrawal began.
Scientists in the US and Europe looked at tree ring data to determine the climate in eastern Europe at the time. They found in the late 1230s the weather was warm and dry, ideal for moving large armies on horseback.
On the other hand, 1241 and 1242 were cold and wet. Initially, this was good for the Mongols -- the freezing of the Danube allowed their armies to cross with ease.
But when spring came, the land turned to swamp.
"Small climatic fluctuations resulted in marshy terrain across the Hungarian plain, which led to reduced pastureland and decreased mobility, as well as hampering the military effectiveness of the Mongol cavalry," the scientists found.
The changeable weather was unsuitable for an occupying force which depended so heavily on horses and the researchers say this, combined with political upheaval back home and a growing Islamic influence on the Mongol civilisation, prompted them to give up on Europe.