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Altan Khan of the Tümed and Khutugtai Sechen Khung-Taiji of the Ordos, the greatest Mongol kings of the later sixteenth century, were converted to Tibetan Buddhism in 1576 by the Third Dalai Lama after dallying with many different faiths. This single event made Buddhism return to political relevance in Inner Asia after centuries of being marginalized by Islam.

ITTL, the two khans convert to Islam. Alternately, Esen Taiji, the ruler of the Oirats in the early fifteenth century who briefly held hegemony over the Mongolian steppes, is serious about converting to Islam (IOTL he "converted" when he married a Muslim Genghisid princess, but it was just a show since Muslim women can't marry infidel men).

What matters is that the Mongols become Muslim and not Buddhist.

In the early seventeenth century, Nurhaci begins state-building among the Jurchens as OTL. It is well-known that Nurhaci's empire was profoundly influenced by Mongol traditions (the Manchu banners are clearly inspired by Mongol thousands and myriads). ITTL, where all the Mongols are Muslim, Nurhaci and his fellow Jurchen chiefs convert to Islam too.

When the Manchus conquer China as IOTL, they keep Islam (along with the Manchu language, Manchu dress, etc) as a marker of privileged Manchu status.
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