After years of deepening mistrust and simmering tensions, ties between the United States and China appeared poised for a modest rebound following the meeting of the two nations’ leaders at a summit last November and recent efforts by Beijing to stabilize its relations with the world. A visit by Secretary of State Antony J. Blinken to Beijing this weekend was expected to build on that progress. All it took was a balloon to upend everything. The discovery of what American military officials called a “high-altitude surveillance balloon” over Montana this…