The expansion into Europe was halted by one death, Uggedai Qa Quan’s, in Karakorum in December 1241. It took until April for the Yam messenger to arrive with the news.
Under the Yassa [Jasagh], Chinnghis Qa Quan’s code of laws, all members of the Royal family and all ranking senior generals to to gather, as soon as possible, in Mongolia, to elect a successor. Present in Hungary were Batu, Buri [Jochi’s sons], Guyuk [Uggedai’s son], Kadaan [Chagatai’s son], and Mongke [Tolui’s son], as well as Subodei. Each had brought his personal army [except Subodei] on campaign. Each was determined to serve his own interests [again, except Subodei, and in this case, Batu] at the Kuriltai. They all left for Mongolia. That's what spared Europe.
The Empire was being ruled by a woman, Uggedai’s widow, Toregine, as regent. She engineered Guyuk’s election, despite Uggedai’s preference for a grandson Siremun.
Since Guyuk and Batu had had a major falling out on the campaign in the west, Europe was spared an immediate follow up invasion because Batu had to watch his eastern border. In fact, Guyk died in 1248 on his way to attack Batu. He was succeeded by Mongke, son of Tolui, through an alliance between Batu and Mongke’s mother, Sorghetani Beki.
“The Devil’s Horsemen””
Excellent book.