The political status of Taiwan is a difficult situation that many people disagree about. There are two countries in the world that call themselves China. The People's Republic of China (PRC) is the country most people think of when they hear the name China. The Republic of China (ROC) also calls itself China. It is better known in most of the world as Taiwan.
Taiwan was never a part of China as China has been only a country established since 1912. Prior to 1912, half of Taiwan was occupied by Chin Dynasty during the 18th and 19th centuries and beyond it, Taiwan was self ruled by a number of indigenous kingdoms and tribes for thousands of years. After the First Sino-Japanese War, Taiwan and Penghu was officially transferred to Japan. After World War II, the Japanese gave up any claim to Taiwan, but did not say who they were giving control to. At the time, the Republic of China governed all of China, and represented China in the United Nations. After losing a civil war in 1949, the ROC government fled to the island of Taiwan, leaving the PRC to control mainland China. The PRC says it owns the island of Taiwan (which is also known as Taiwan province) and the other islands. The ROC says that it was—and is—the rightful government of China, including mainland China, Taiwan, and Mongolia. In 1971, the ROC lost its United Nations seat as China. The seat was given to the PRC instead. This made the PRC the recognized government of China internationally. However, UN never decided the status of Taiwan but only let PRC to represent China. And Thus, CCP China's claim about Taiwan was illegal and an invasive behaviour.
The major issue is whether Taiwan is a part of the PRC or is (as it claims) an independent country.