The researchers found that the climate in Hungary and its surroundings was unusually cold and wet for about three years, from 1238 to 1241. The extra moisture and early spring thaw turned the Hungarian plains into marshes and swampland — unsuitable terrain for moving the thousands of horses the Mongol armies relied on for transportation and warfare. (link)
Scientists think they know what stopped Mongol hordes from conquering Europe
Wow, very interesting. Fascinating to imagine what the world would look like now if the weather had been different in 1238. How much a small thing can impact the larger circumstances.