Whether the U.S. president-elect meant to jump into the generational fight between China and Taiwan remains an open question. But by speaking to Taiwanese President Tsai Ing-wen, Trump upended four decades of American foreign policy and engaged China directly on the issue of Taiwan, which Beijing has threatened to reclaim by force if necessary. No American president or president-elect has publicly spoken to Taiwan's leader since the U.S. ended their formal diplomatic relationship in 1979.