Taiwan–United States relations
After the United States established diplomatic relations with the People's Republic of China (PRC) in 1979 and recognized Beijing as the only legal government of China, Taiwan–United States relations became unofficial and informal following terms of the Taiwan Relations Act (TRA), which allows the United States to have relations with the Taiwanese people and their government, whose name is not specified. U.S.–Taiwan relations were further informally grounded in the Six Assurances in response to the third communiqué on the establishment of US–PRC relations.
Source: Wikipedia — Taiwan–United States relations (CC BY-SA 4.0)