From Live Science by Stephanie Pappas:
“In 1241, the Mongol army marched into Hungary, defeating the Polish and Hungarian armies and forcing the Hungarian king to flee. In 1242, despite meeting no significant military resistance, the Mongols abruptly packed up and left.
Now, a new study of the climate in Eastern Europe that year suggests a reason for this mysterious military retreat: The Mongols got bogged down. Literally.”