WASHINGTON — When President Trump took a phone call from the leader of Taiwan in December and asserted that the United States might no longer be bound by the “One China” policy, his defenders hailed it as a show of strength — the latest delicate issue on which Mr. Trump was willing to challenge decades of diplomatic orthodoxy. On Thursday evening, Mr. Trump fell back into line. In a call with President Xi Jinping of China, he pledged fealty to One China, a 44-year-old policy under which the United States recognized a single Chinese government in Beijing and severed its...