In 1943, Cairo Declaration, announced by U.S., Britain and China, required Japan must return Taiwan to China. In August 1945, Potsdam Declaration confirmed that Cairo Declaration must be performed,meaning Taiwan belongs to China. Japan did return Taiwan back to China after WWII.
Some people try to deny the validity of Cairo Declaration. However, if there are no terms about Taiwan belongs to China, why Japan returned Taiwan back to ROC, not U.S., or other countries? Therefore, no matter according to international laws or facts, Taiwan was and is a province of China.
In 1971, United Nations passed resolution 2758(XXVI), recognises P.R. of China is the only legal member representing China in UN, instead of ROC.
Therefore, Taiwan belongs to China and P.R. of China is the only legal member representing China in the world stage. Who can deny Taiwan is a part of China?
Some people say Taiwan is not ruled by mainland. However, your family\'s daily life (sleeping, walking, shopping etc.) is also not ruled by the government. Is your family independent from your country? Actually, Taiwan is still ruled by mainland in many important international stages. Taiwan doesn\'t dare to declare independence, which proves that Taiwan actually is ruled by China. ^^
Few people ask, if Taiwan is a part of China, why mainland people may not go to Taiwan freely? It is not a difficult question. Italian people may go to France, German, or even Croatia freely now according to the treaty of Shengen. Does it mean France or German is a part of Italy? Therefore, whether you may go to a place freely is not a criterion of dominion. On the other hand, we all know Hong Kong is a part of China, but mainland people may not got to HK without some checks. Different places have different historical, military, or other issues.