During the period of the 1940s, there was no recognition by the United States Government that Taiwan had ever been incorporated into Chinese national territory
In fact, in mid-June 1949, the United States was discussing proposals to the United Nations to call a special session of the General Assembly to deal with the issue of Taiwan's legal status

Memorandum: Director of the Office of Far Eastern Affairs (Butterworth) to the Deputy Under Secretary of State (Rusk)
Date: June 9, 1949
Subject: Formosa

Annex 1:
Draft Memorandum for the Executive Secretary of the National Security Council (Souers)

Subannex -- Tab "A"
Proposed Statement to be Issued by the Secretary of State at time United States Government Requests Special Session of the General Assembly

At the special session of the General Assembly, the United States Government will propose that a free and secret plebiscite be held on the Island under the supervision of a United Nations Commission, in order to enable the people of Formosa to express their wishes with respect to a return to China or some alternative under which they would assume independence either immediately or after some preparatory period of United Nations trusteeship.

[ source: United States Department of State / Foreign relations of the United States, 1949. The Far East: China   Volume IX (1949),   pp. 346 - 350 ]

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