shifted to a wetter and colder environment. That, in turn, caused flooding of the formerly dry grasslands and created a marshy terrain. Those conditions would have been less than ideal for the nomadic Mongol cavalry and their encampments, reducing their mobility and pastureland, curtailing their invasion into Europe west of the Hungarian plain, and hastening their retreat. The true reasons for the Mongol withdrawal are not fully known, but numerous plausible explanations exist. The Mongol invasion had bogged down into a series of costly and frustrating sieges, where they gained little loot and ran into stiff resistance. They had lost a large number of men despite their victories (see above). Finally, they were stretched thin in the European theater, and were experiencing a rebellion by the Cumans (Batu returned to put it down, and spent roughly a year doing so). Others argue Europe's bad weather had an effect: Hungary has a high water table so it floods easily. An analysis of tree rings there found that Hungary had cold wet weather in early 1242, which likely turned Hungary's central plain into a huge swamp; so, lacking pastures for their horses, the Mongols would have had to fall back to Rus' in search of better grasslands. Regardless of their reasons, the Mongols had completely withdrawn from Central Europe by mid-1242, though they still launched military operations in the west at this time, most notably the 1241–1243 Mongol invasion of Anatolia. Batu specifically decided against attending the kurultai in favor of staying in Europe, which delayed the ceremony for several years. The historian Jack Weatherford claims that European survival was due to Mongol unwillingness to fight in the more densely populated German principalities, where the weather affected the glue and sinew of the Mongol bows. However, a counter to this assertion is that the Mongols were willing to fight in the densely populated areas of Song China and India. Furthermore, the Mongols were able to conquer Southern China which is located in a tropical climate zone and would have received far more rainfall and humidity than anywhere in Europe. The territory of Western Europe had more forests and castles than the Mongols were accustomed, and there were opportunities for the European heavy cavalry to counter-attack. Also, despite the steppe tactics of the Avars and early Hungarians, both were defeated by Western states in the 9th and 10th centuries, though many states conquered