Why did the Khalkha Mongols seek Qing protection?

The Khalkha Mongols sought protection from the Qing dynasty because they were invaded by the Dzungar Oyrats, a nomadic people led by the ambitious ruler Galdan Boshigt Khan.1

Following his conquest of East Turkistan, Galdan Boshigt invaded the territories of the Khalkha people, who lived in what is now Outer Mongolia. Faced with this military aggression, the Khalkha fled in large numbers to Inner Mongolia to escape the Dzungar forces and formally pledged their allegiance to the Kangxi emperor at Doloon Nuur in 1691. This alliance ultimately led to the Qing military campaign that defeated the Dzungars and integrated Outer Mongolia into the Qing empire.1