Soon after gaining independence, the central government led by U Nu faced several armed rebellions. The most serious was the Chinese Nationalist KMT invasion of Shan State in 1950. Driven out by the Chinese Communist forces, Nationalist KMT troops  planned to use the region east of the Salween river as a base from which to regain their homeland. In March 1953, the KMT forces with US assistance were on the verge of taking the entire Shan State, and within a day's march of the state capital Taunggyi. The Burmese army drove back the invaders east across the Salween. The KMT units remained there until 1961 when some 20,000 troops of the People's Liberation Army and 5,000 troops of the Burmese Army defeated them in a joint operation.. How many years did the KMT forces remain located east across the Salween after being driven back by the Burmese army?