From a formal legal standpoint, the United States never ceded possession of Taiwan [via Simon World], which it took from Japan in 1945, to the Nationalist government. It’s still ours!
This raises all kinds of interesting issues, if you take these sorts of things seriously (with international law, it’s hard, because nobody really pays that much attention to the paperwork, do they?). The last time we gave away something we took from the Japanese, instead of making it an independent state (as most of its inhabitants wanted) we gave it back: Okinawa. Of course, we have a different relationship with China….