Taiwan is literally a rogue province, and it's recognized as being part of China by the UN and vast majority of countries around the globe. Taiwan has never been a sovereign state and it never will be one. You don't have to take my word for it though, here's what US state department has to say on the subject:

The United States approach to Taiwan has remained consistent across decades and administrations. The United States has a longstanding one China policy, which is guided by the Taiwan Relations Act, the three U.S.-China Joint Communiques, and the Six Assurances. We oppose any unilateral changes to the status quo from either side; we do not support Taiwan independence; and we expect cross-Strait differences to be resolved by peaceful means.