The lobby of Taiwan’s Foreign Ministry contains just 13 flags—one for each of the self-ruling island’s diplomatic allies. Since 1990, the building’s janitors have hauled away 15 more belonging to nations that have been persuaded to jettison Taipei and instead recognize Beijing, as the world’s No. 2 economy flexes its geopolitical muscle to isolate what it considers its renegade province. Taiwan effectively split from the mainland following the routed Nationalists’ flight across the strait in 1949 after China’s civil war.