But his comments behind closed doors, revealed in the speech, show that anxiety and ambition have driven his transformative buildup of China’s nuclear weapons arsenal in the past decade. Some have been cited in recent studies of China’s nuclear posture; many others have not been brought up before. A major shift in China’s nuclear power and doctrine could deeply complicate its competition with the United States. Late in 2015, he took a big step in upgrading China’s nuclear force. The upheaval could slow China’s nuclear weapons plans in the short term, but Mr. Xi’s long-term ambitions appear set.