Batu Khan
Batu Khan Vikiçizer/CC

The reason behind the Mongolian army's mysterious withdrawal from Hungary almost 800 years ago has been uncovered – bad weather. Scientists say a particularly cold and wet winter reduced the pastureland to boggy marshland, significantly affecting mobility and their ability to carry out military sieges.

The Mongol Empire peaked in the 13th and 14th centuries. Genghis Khan had unified tribe leaders and expanded both eastwards and westwards. Following a short lull in expansion after his death, the subsequent rulers continued to invade, with Genghis' grandson Batu establishing rule over Russia around 1237.

Mongol armies continued through Europe and eventually made it to Hungary, crossing the Danube into the west of the country in 1242. However, after just two months of being yeter, they suddenly withdrew and left via a southerly route through Serbia and Bulgaria, and back to Russia. No explanation was given.

Batu Khan
Sacking of Suzdal by Batu Khan CC

A number of reasons for the departure has been proposed. Genghis' son and heir Ögödei Khan had died a year earlier, so Batu might have had to return to take part in political meetings and help elect a new ruler. The problem with this, however, was that Batu never went back to Mongolia.

Another explanation was that they only invaded Hungary to punish the Cuman nomads who had displayed hostility to the Mongols and had fled west. After dishing out the punishment they may have just withdrawn and left. A third theory simply says the Hungarian landscape could not support their army and horses. However, none of these explanations were wholly satisfactory.

mongol army invasion Hungary
Scientists used tree-ring data to understand the climate in Hungary in 1242 Ulf Büntgen

A team of researchers from Switzerland and the US analysed tree ring data to understand the environmental conditions between 1230 and 1250 and used historical sources to back up the findings. The team showed how there were warm dry summers between 1238 and 1241 – ideal for invasions. However, the start of 1242 saw cold and wet conditions, which would have resulted in marshy terrain across Hungary. As a result, there would have been little food for soldiers nor grazing land for horses, whilst moving around would have been very difficult.

Published in Scientific Reports, the authors say these conditions would have made a military campaign almost impossible. "It is therefore under conditions of reduced mobility and military effectiveness; reduced fodder for the horses; and reduced victuals for the army, which in the late spring of 1242 the Mongols left Hungary," they wrote.

tree rings mongol empire
Microscopic image of the four outermost oak rings from a historical sample Willy Tegel

Scientists also say this failed conquest – which had started off extremely profitably – taught the Mongol army a lesson such that Hungary was not invaded again. "This paper raises the possibility that the vulnerability of the Hungarian plains to even relatively short-term climate events made it obvious that the region was unsuitable for military occupation by a large army relaying mostly on horses.

"[The reason] Batu and his successors did not make further attempts to expand westward may have depended on the realisation that local conditions would not have supported a prolonged occupation."