The Mongols were arguably at their apical stage of military power when they made their forays into Eastern Europe after 1240 AD. And by 1242 AD, they managed to heavily defeat Poland, Hungary, an assortment of knightly orders; and then proceeded on to invade Croatia. But almost like a miracle, the Mongol forces surprisingly decided to halt their push into Central Europe, and the main body of troops antithetically made their way back into Asia, thus leaving behind a trail of destruction and sordid legends. Now the most common hypothesis for such a hasty retreat on the part of the Mongols is usually attributed to the sudden death of Ögedei Khan, the Supreme Khan of the Mongols, who possibly met his demise after binge drinking at a hunting trip. However a recent research (headed by Nicola Di Cosmo, a historian at Princeton University) has shed some new light into this odd affair. And according to the study, the anticlimactic Mongol retreat was probably fueled by the abrupt weather effects on the local eco-system, rather than some impractical ideal instigated by a distant leader’s death.

Nicola Di Cosmo has talked about how the Mongols might have fielded almost 130,000 men in Europe, and they were possibly accompanied by half-a-million horses, circa 1240 AD. By April of 1241, they soundly orchestrated a string of defeats on Poland and its allies. And by the early months of 1242, the Mongols crossed into Hungary via the frozen expanses of Danube. Now historically in the very same year, the Mongols hastily retreated back, without incurring any major loss on the European battlefields – thus signifying a mystery related to their logistical scope. But as the researchers have deduced (with the aid of data gathered from tree rings), the particularly nasty winter rather helped in bringing about a wet spring in Hungary. This might have resulted in the grasslands of Hungary turning into marshes, which would have made them ineffective as grazing grounds for the very high number of Mongol ponies and horses.

Microscopic view of four oak rings used for ‘recording’ the weather of 1241 and 1242 in Eastern Europe.

Now from the sequential perspective, the hypothesis does make sense. This is because the death of Ögedei Khan didn’t really force Batu Khan, the commander of the Mongols forces in Europe and the heir-apparent (being the grandson of Genghis Khan), to go back to Mongolia; rather he played his political role in Russia and founded the Golden Horde. Furthermore, the analysis of tree rings offer an unbiased insight into the climatic ambit of a region – as can be extrapolated from a tree’s summer growth and winter quiescence. The study goes on to mention –

Documentary sources and tree-ring chronologies reveal warm and dry summers from 1238–1241, followed by cold and wet conditions in early-1242. Marshy terrain across the Hungarian plain most likely reduced pastureland and decreased mobility, as well as the military effectiveness of the Mongol cavalry, while despoliation and depopulation ostensibly contributed to widespread famine. These circumstances arguably contributed to the determination of the Mongols to abandon Hungary and return to Russia. While overcoming deterministic and reductionist arguments, our ‘environmental hypothesis’ demonstrates the importance of minor climatic fluctuations on major historical events.

Interestingly enough, according to Di Cosmo, it was possibly the very effects of the warm and wet weather patterns from 1211 – 1225 AD that gave the Mongols access to ample grazing grounds for their horses, thus resulting in spurts of successful invasions and forays during the period. Now of course, beyond just climatic factors, one cannot also ignore the social, military and organizational aspects of these advanced (and hardy) nomads that made them the most powerful military force of the 13th century.

The study was originally published in the journal Scientific Reports.