A new Mongol invasion of Europe forced thousands of Cumans to flee from the steppes in the summer of 1237. Istvan Vassary states that after the Mongol conquest, "A large-scale westward migration of the Cumans began." Certain Cumans also moved to Anatolia, Kazakhstan and Turkmenistan. In the summer of 1237 the first wave of this Cuman exodus appeared in Bulgaria. The Cumans crossed the Danube, and this time Tsar Ivan Asen II could not tame them, as he had often been able to do earlier; the only possibility left for him was to let them march through Bulgaria in a southerly direction. They proceeded through Thrace as far as Hadrianoupolis and Didymotoichon, plundering and pillaging the towns and the countryside, just as before. The whole of Thrace became, as Akropolites put it, a "Scythian desert."