The United States established diplomatic relations with China in 1979. In order to maintain those relations, the United States has officially refused to recognize Taiwan as an independent nation in order to appease Chinese officials who claim that Taiwan belongs to them.
President elect Donald Trump today spoke with Taiwanese president Tsai Ing-wen and congratulated her on her victory earlier this year. Considering that China does not recognize the legailty of Tsai Ing-wen’s political position, this move clearly upset Chinese officials.
Trump officials declined to explain whether president-elect Trump made the call intentionally, knowing it would upset Chinese officials, or if he was simply unaware of our 37-year-long official stance to not recognize the president of Taiwan.
source: http://www.cnn.com/2016/12/02/politics/donald-trump-taiwan/index.html