What if?

Batu, a grandson of Ginghis [Khan], soon conquered the Crimea, then ravaged what is now Bulgaria as well as fourteen Russian cities, turning their shattered remnants into vassal states. Next he turned his atention to Europe, with the objective of reaching "the ultimate sea." The Mongols under General Subatai divided into three groups, conquered Poland and Hungary, and swept into Austria, where they prepared for a probe into the heart of Europe in 1241. At that moment, Ogotai Khan [son of Ginghis] died. Batu Khan was a potential candidate for Great Khan, so he withdrew his forces to the steppes. In the event, he was not chosen and devoted his efforts to consolidating his conquests around the Urals. He held sway over the Cuman steppes and over various Russian kingdoms and never returned to the scene of his former conquests.

Batu Khan's withdrawal coincided with the return of cooler, wetter conditions, which brought improved pasturage to the steppes. His kingdom flourished during generations of good pasturage, when warfare died down. Although Batu always maintained ambitions of returning west, good grazing conditions at home allowed his people to pasture a huge territory from the Volga-Don to Bulgaria. There were no incentives for ambitious conquests when grazing was plentiful and trade flourished with lands to the south.

But what would have happened if the climatic pendulum had not swung, and if droughts had intesified on the steppe? To judge from earlier centuries, warfare and restless movement would have continued, and almost certainly, Batu Khan and his generals would have returned to the west. His spies had already given him a clear picture of the kingdoms that confronted them, and of their armies with their heavily armored knights, who had proved no match for Mongolian archers and horsemen. He would have followed his original plans, drawn up with General Subatai: invade Austria and destroy Vienna first, then move against the German principalities before turning his attention to Italy. If all went well, he would have then marched into France and Spain. Within a few years, perhaps as early as 1250, Europe would have become part of a huge western Mongolian empire.

--Brian Fagan, The Great Warming: Climate Change and the Rise and Fall of Civilizations