He wasn't the complete ruler of the Mongols. Another Mongol leader actually raided China during the same time and was repulsed. He was Jasaghtu Khan, who was repulsed by Qi Jiquang, already a hero for cleaning up the wokou Japanese pirates.Had the Ming built a solid and long term relationship with the Dayanid
dynasty of Altan, the Manchus would never have been able to prevail and conquer China. First and foremost the formidable Mongol cavalry
would serve and protect the Ming dynasty.
The Mongols allied themselves willingly to Nurhachi? Not. Like the Koreans, Nurhachi defeated the Mongols in a series of bloody battles, before integrating both into his armies. Later the Qing Dynasty sent armies into the Mongol and Turkish proper, decimating both peoples into the marginal state they are today. For that, Mongol and Turks deeply hated the Manchus. After the Manchus fell in 1911, Mongol and Turkish minorities were out to slit the necks of anyone who remotely looked like a Manchu (bald wearing a pony tail), and Manchu hairstyles quickly disappeared if you don't want your throat slit.