Taiwan was permanently ceded to Japan. Taiwan and the rest of the Japanese owned territories prior to WW2 in the Asia Pacific were trusted to the Ally Powers according to the UN charter.
UN Charter, Chapter 7, Article 77
The trusteeship system shall apply to such territories in the following categories as may be placed thereunder by means of trusteeship agreements:
- territories now held under mandate;
- territories which may be detached from enemy states as a result of the Second World War; and
- territories voluntarily placed under the system by states responsible for their administration.
The purpose of trusteeship is to:
UN Charter, Chapter 7, Article 76
b. to promote the political, economic, social, and educational advancement of the inhabitants of the trust territories, and their progressive development towards self-government or independence as may be appropriate to the particular circumstances of each territory and its peoples and the freely expressed wishes of the peoples concerned, and as may be provided by the terms of each trusteeship agreement;
As such, the trust territory are entitled to has to follow Chapter 1, Article 1, part 2 of UN’s charter, and deserve right to self-determination.
The US as the primary victor is the trustee, and it merely relegated the right to administration to the ROC. The ROC had no right unilaterally annex Taiwan, declare Taiwan a province, conscript the people of Taiwan, or even to tax the people of Taiwan to be used in China.
It certainly had no right to declare martial law over the trust territory, enact a police state and practice white terror over the population of the trust territory.
The UN charter was signed in San Francisco on 26 June 1945. That was before Japan’s surrender, and the US navy didn’t ferry KMT troops over to Taiwan until that October.