Scientists have finally figured out what kept the Mongol Hordes from taking over Europe

Genghis Khan, a strong tribal leader from northern Mongolia, started to take over the world in 1206. Asia was taken over by the khan’s cruel tactics and loyal army.

One country after another fell to the overwhelming power of the Mongol Empire, which eventually reached from the east coast of China to the west coast of Russia. A series of successful forays in Hungary and Poland made even Europe seem within reach of conquering.

But all of a sudden, this wave of victories in Europe stopped. Almost as soon as they set their sights on Austria, the Mongols turned around and went back to Asia.

Historians could only guess why until now, since written accounts from the point of view of Mongol military leaders are sparse. But a new study published in the journal Scientific Reports looked at tree rings to figure out why the horde left central Europe so quickly.

This wooden chronicle showed that there was a long period of cold and wet weather. According to a press release, this led to less pastureland and less freedom of movement, and it also hurt the Mongol cavalry’s ability to fight.

Here’s how the Mongols got to be in charge, as well as how a change in the weather may have forced them to stop a scary war of attrition.

Ahead of Hungary

When Genghis Khan died in 1227, he left his son, Ogodei, a territory that extended from northeast China to the Caspian Sea, just north of modern-day Iran. It was an incredible 11 million square miles in size (28 million square km).

“Genghis Khan conquered more than twice as much as any other man in history, based on the number of people he killed, the number of countries he took over, or the amount of land he took over,” writes historian Jack Weatherford in his book Genghis Khan and the Making of the Modern World.

After Genghis died, his son Ogodei Khan continued what his father had started. The khanate grew to the east and west, taking over the rest of northwest China and pushing into Russia. A wet period helped the Mongol armies bring thousands of horses across the Gobi, which is the largest desert in Asia.

By 1240, Kiev had been burned down, and the horde was moving quickly toward the west. Their cavalry and siege tactics were destroying the cities of Europe, and maybe even more important, they brought Chinese gunpowder with them.

The huge Mongol army arrived in Hungary in March 1241 after a long string of wins. King Bela IV ran away from his palace in Pest, which is now called Budapest, and Ogodei’s armies killed about 1 million Hungarians, including soldiers, priests, nobles, knights, and peasants. It was one of the most bloody losses in the Middle Ages.

In December of 1241, it was a surprise that Ogodei Khan died. Some historians say that Batu, Ogodei’s nephew who was in charge of the campaign in the west, went back to Karakorum, the capital of the Mongols, to vote for a new leader.

But Batu never went back to Mongolia. He stayed in southern Russia and ruled the Golden Horde from there. At the same time, Toregene, Ogodei’s wife, became the Great Khatun.

An abrupt end

Everything changed the next year. The horde suddenly went south, through what is now Serbia, and then turned around and went back through Russia. Even though other khans did raids on European cities from time to time, the main war campaign was over.

There are a few ideas about why the army left the western front, but the authors of the new paper argue that none of them are good enough to fully explain the change.

The authors took samples of wood from five different parts of Eurasia to find out what the weather was like when the Mongols were at their most powerful.

Trees are very sensitive to even small changes in the weather. When it rains, they add thick layers of bark to their trunks. In dry years, the rings are thinner, which shows that the tree didn’t get enough water.

From 1238 to 1241, they found that the weather in Hungary and the area around it was unusually cold and wet. The early spring thaw and extra rain turned the Hungarian plains into marshes and swamps, making it hard for the Mongol armies to move the thousands of horses they used for transportation and battle.

They say that 1242, the last year of the campaign in Eastern Europe, was very wet. This caused crops to go bad, so there was even less food for the khan’s hordes. Later, there was a famine that killed a lot of people in the area.

The authors say it’s likely that Ogodei’s leaders chose a southern route because it was drier there. This took them away from Europe.

What did the Mongols do after that? When Ogodei Khan died, Genghis’s sons and grandsons fought over who was in charge. This broke the Mongol Empire into pieces that never came back together.

His family, on the other hand, went on to start new dynasties in India, China, Persia, and Siberia. Mongols still live in China’s Inner Mongolia Autonomous Region and modern-day Mongolia, where Genghis Khan’s face is on money, vodka, and cigarettes, and his name is on the international airport in Ulaanbaatar.

As scientists learn how to look at the climate record in more detail, we learn more about how the climate has changed over time. Unusual weather probably helped Polynesians spread out across the South Pacific, caused an ancient metropolis to fall in Mexico before it was colonized, and helped Attila the Hun terrorize the Roman Empire 800 years before Genghis Khan.

The authors come to the conclusion that their study of the Mongolian withdrawal from Hungary “shows how even small changes in the weather can affect a historical event.”

It also shows us a lesson about the future of our climate: it only takes a few degrees to change the course of human history.