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It has mystified historians ever since. After a string of major victories, the Mongol army suddenly retreated from central Europe in 1242.

Some scholars claim Mongolian politics forced the withdrawal, while others credit the strength of fortified towns in present-day Hungary and Croatia. But Europe could have been rescued by its own bad weather, an analysis of tree rings and historical documents concludes.

The Mongol cavalry fed its horses on the grass of the Eurasian steppe, says of Princeton University, one of the study鈥檚 authors. A warm climate in the early 1200s and this, in turn, helped the Mongols extend their empire into Russia, he says.

In 1241, the Mongol army reached the plains鈥� western limit in Hungary. Led by Genghis Khan鈥檚 grandson Batu, the Mongols crushed the Polish and Hungarian armies on open, flat terrain that suited their mobile warfare tactics.

鈥淭hey were familiar with that environment,鈥� says Di Cosmo. 鈥淲hat they didn鈥檛 know is how prone to flooding that particular area was.鈥�

Huge swamp

Compared with the rest of the steppe, Hungary has a high water table so it floods easily.

Analysing tree rings in the region, Di Cosmo and his colleagues found that Hungary had a cold, wet winter in early 1242. This probably turned Hungary鈥檚 central plain into a huge swamp.

Historical documents the team studied back up this claim, recording, for example, that melting snows kept the Mongol army from attacking a Hungarian castle surrounded by marshes.

Lacking pasture for its horses, the Mongols fell back to drier highlands and then to Russia in search of better grass.

While climate wasn鈥檛 the only factor in the reteat, it would be a mistake to ignore it, says Di Cosmo. 鈥淚t鈥檚 like saying the winter in Russia had no effect on Napoleon鈥檚 army,鈥� he says.

of Pennsylvania State University, University Park, says the study is interesting, but he warns against over-interpreting the influence of climate on historic events. 鈥淚鈥檓 sceptical that such 鈥榗limate determinism鈥� holds nearly as universally as some authors seem to think,鈥� he says. The changes in weather the study reported seemed 鈥渕odest鈥�, he says.

But of the University of Maine in Orono says that the study steered clear of determinism, taking into account all potential factors. 鈥淚 think it鈥檚 convincing,鈥� he says. 鈥淭he previous explanations of the Mongol withdrawal didn鈥檛 add up.鈥�

Horse logistics limited the Mongols, Putnam says. 鈥淭hey were incredibly technologically savvy, but they got into a place where horses just didn鈥檛 do well.鈥�

Putnam says that natural weather records like tree rings have much more to tell us about the history of premodern civilisations, which depended heavily on environmental conditions. 鈥淚t鈥檚 just an incredible archive.鈥�

Read more: Five civilisations that climate change may have doomed