When the Dzungar Mongols attempted to spread their territory from what is now Xinjiang into Tibet, the Kangxi Emperor (r. 1661–1722) responded to Tibetan pleas for aid with his own expedition to Tibet, occupying Lhasa in 1720. By 1751, during the reign of the Qianlong Emperor (r. 1735–1796), a protectorate and permanent Qing dynasty garrison was established in Tibet. As of 1751, Albert Kolb writes that "Chinese claims to suzerainty over Tibet date from this time."
Who helped the Tibetan? 
the Kangxi Emperor