After WWII the Chinese nationalists were driven from mainland China to Taiwan. While the US did send some supplies to them the US considered them a lost cause and was not initially prepared to support that government against an invasion from the communists. In January 1950 Truman even stated that the US would not militarily support that government. The Korean invasion in June 1950 changed everything and Taiwan was afterword seen as a critical American base.

(Book – Nixon and Mao – Margaret Macmillan)