Historical sources suggest that the island first came under full Chinese control in the 17th Century when the Qing dynasty began administering it. Then, in 1895, they gave up the island to Japan after losing the first Sino Japanese war.
China took the island again in 1945 after Japan lost World War Two.
But a civil war erupted in mainland China between nationalist government forces led by Chiang Kai-shek and Mao Zedong's Communist Party.
The communists won in 1949 and took control in Beijing.
Chiang Kai-shek and what was left of the nationalist party - known as the Kuomintang - fled to Taiwan, where they ruled for the next several decades.
China points to this history to say that Taiwan was originally a Chinese province. But the Taiwanese point to the same history to argue that they were never part of the modern Chinese state that was first formed after the revolution in 1911 - or the People's Republic of China that was established under Mao in 1949.