It’s a reference to the Chinese civil war. When it was clear the communists were going to win the head of the nationalist Kuomintang party Chiang Kai shek fled to Taiwan and established the modern republic of China (Taiwan) in hopes of eventually returning and retaking the mainland after communism collapsed or the US decided to get involved. After they left mao mopped up the remaining nationalist forces and established the people’s republic of China, both of them claim to be the legitimate government of both Taiwan and the mainland.