The charter of the United Nations says:
The Purposes of the United Nations are:
- To maintain international peace and security, and to that end: to take effective collective measures for the prevention and removal of threats to the peace, and for the suppression of acts of aggression or other breaches of the peace, and to bring about by peaceful means, and in conformity with the principles of justice and international law, adjustment or settlement of international disputes or situations which might lead to a breach of the peace;
- To develop friendly relations among nations based on respect for the principle of equal rights and self-determination of peoples, and to take other appropriate measures to strengthen universal peace;
And in Article 2, section 4:
All Members shall refrain in their international relations from the threat or use of force against the territorial integrity or political independence of any state, or in any other manner inconsistent with the Purposes of the United Nations.
So if Taiwan is a "state", the UN charter says that no UN member may deprive it of its right to self-determination, and in particular may not use the threat or use of force to do so.
This begs the question of whether Taiwan is a "state". As the charter itself does not define this term, let's consult what the Encyclopædia Britannica says about states in international law:
The accepted criteria of statehood were laid down in the Montevideo Convention (1933), which provided that a state must possess a permanent population, a defined territory, a government, and the capacity to conduct international relations.
Taiwan meets all these criteria: The defined territory is the island of Taiwan, permanent home to 23 million people, a government with all the usual ingredients, which demonstrates its capability to conduct international relations by maintaining the 31st largest diplomatic network in the world with 110 offices.
Appendix: Recognition of statehood
Recognition is a process whereby certain facts are accepted and endowed with a certain legal status, such as statehood, sovereignty over newly acquired territory, or the international effects of the grant of nationality. The process of recognizing as a state a new entity that conforms with the criteria of statehood is a political one, each country deciding for itself whether to extend such acknowledgment. Normal sovereign and diplomatic immunities are generally extended only after a state’s executive authority has formally recognized another state (see diplomatic immunity). International recognition is important evidence that the factual criteria of statehood actually have been fulfilled. A large number of recognitions may buttress a claim to statehood even in circumstances where the conditions for statehood have been fulfilled imperfectly (e.g., Bosnia and Herzegovina in 1992). According to the “declaratory” theory of recognition, which is supported by international practice, the act of recognition signifies no more than the acceptance of an already-existing factual situation—i.e., conformity with the criteria of statehood. The “constitutive” theory, in contrast, contends that the act of recognition itself actually creates the state.