The correct answer is "the United States support of Taiwan instead of mainland China during WW II."
Disagreement over the sovereignty of Taiwan can be traced to the United States' support of Taiwan instead of mainland China during WW II.
US President Harry S. Truman sent a fleet of ships to Taiwan in 1949 to protect it from Red China.
As Taiwan decided to separate from years of Chinese dominion, conflicts were expected between the two countries. That is why United States President Harry S. Truman sent a fleet of ships to Taiwan in 1949 to protect it from Red China's aggressions. The US knew that China would disapprove of the separation of Taiwan and was very interested in maintaining control over this island because of economic and political reasons.