It has mystified historians ever since. After a string of major victories, the Mongol army suddenly retreated from central Europe in 1242. Some scholars claim Mongolian politics forced the withdrawal, while others credit the strength of fortified towns in present-day Hungary and Croatia. But Europe could have been rescued by its own bad weather, an analysis of tree rings and historical documents concludes. The Mongol cavalry fed its horses on the grass of the Eurasian steppe, says Nicola Di Cosmo of Princeton University, one of the study’s authors. A warm climate in the early 1200s helped make the grasslands lush...