With the fall of the mainland, the United States largely wrote off the Kuomintang and Chiang Kai-Shek as incompetent, corrupt leaders who deservedly lost and was prepared to grant diplomatic recognition to the PRC. Some influential American voices expressed the cautious optimism that the newly established communist regime would be more honest and concerned about the Chinese people's welfare, especially helped by the fact that the Kuomintang deeply distrusted the US and had little in the way of common, shared values. All of this changed rapidly when the PRC intervened in the Korean War, which ruined any chance of normalizing relations with Washington for years. With mainland China now a sworn enemy of the US, the latter extended a cautious olive branch to Taipei under the adage of "the enemy of my enemy is my friend". The victorious Battle of Guningtou by the ROC forces against the communist forces helped to boost morale in the ROC Army and diminished any chance by the communist forces to take Taiwan. Subsequently, the United States Seventh Fleet started to patrol the Taiwan Straits. By the 1950s, Sino-American Mutual Defense Treaty was signed and US provided Military Assistance and Support to the ROC forces. The US Army maintained a garrison force in Taiwan until its withdrawal in 1979. The Sino-American Mutual Defense Treaty was replaced by the Taiwan Relations Act after 1979.