Task: Question Answering
Input (English): Passage: In 1565, the powerful Rinbung princes were overthrown by one of their own ministers, Karma Tseten who styled himself as the Tsangpa, "the one of Tsang", and established his base of power at Shigatse. The second successor of this first Tsang king, Karma Phuntsok Namgyal, took control of the whole of Central Tibet (Ü-Tsang), reigning from 1611–1621. Despite this, the leaders of Lhasa still claimed their allegiance to the Phagmodru as well as the Gelug, while the Ü-Tsang king allied with the Karmapa. Tensions rose between the nationalistic Ü-Tsang ruler and the Mongols who safeguarded their Mongol Dalai Lama in Lhasa. The fourth Dalai Lama refused to give an audience to the Ü-Tsang king, which sparked a conflict as the latter began assaulting Gelug monasteries. Chen writes of the speculation over the fourth Dalai Lama's mysterious death and the plot of the Ü-Tsang king to have him murdered for "cursing" him with illness, although Chen writes that the murder was most likely the result of a feudal power struggle. In 1618, only two years after Yonten Gyatso died, the Gelug and the Karma Kargyu went to war, the Karma Kargyu supported by the secular Ü-Tsang king. The Ü-Tsang ruler had a large number of Gelugpa lamas killed, occupied their monasteries at Drepung and Sera, and outlawed any attempts to find another Dalai Lama. In 1621, the Ü-Tsang king died and was succeeded by his young son Karma Tenkyong, an event which stymied the war effort as the latter accepted the six-year-old Lozang Gyatso as the new Dalai Lama. Despite the new Dalai Lama's diplomatic efforts to maintain friendly relations with the new Ü-Tsang ruler, Sonam Rapten (1595–1657), the Dalai Lama's chief steward and treasurer at Drepung, made efforts to overthrow the Ü-Tsang king, which led to another conflict. In 1633, the Gelugpas and several thousand Mongol adherents defeated the Ü-Tsang king's troops near Lhasa before a peaceful negotiation was settled. Goldstein writes that in this the "Mongols were again playing a significant role in Tibetan affairs, this time as the military arm of the Dalai Lama.". 
Question: Who began assaulting Gelug monasteries?
Output (English): Ü-Tsang king.