What precisely happened to the logistic of crossing Eurasia on horseback to cause Mongol Empire to form in 13th century?
There had been a number of united empires in Mongolia. Xiongnu. Rouran. Göktürk Khaganate. Uyghur Khaganate.
Göktürk Khaganate actually invaded as far west as Crimea and attacking Sassanids (which they lost). The others were confined to East Asia.
Could some butterfly prevent or thwart Mongol invasion of the West, with the result that Mongol Empire never expands past Otrar, like Xiongnu, Rouran and Uyghurs had not? Or indeed that Mongolia remains disunited between squabbling tribes for some more centuries, as it had been since fall of Uyghurs in 9th century?
Could a major cavalry invasion of the west from east, resulting in political control of Near East or Europe from East Asia, take place instead of 129 AD at some other time, like 219 BC or 1619 AD?