China’s rise and stepped-up involvement in Southeast Asia have prompted a blend of anticipation and unease among its smaller neighbors. The stunning growth of China has yanked up the region’s economies, but its militarization of the South China Sea and dam building on the Mekong River has nations wary about Beijing’s outsized ambitions. Southeast Asians long felt relatively secure, relying on the United States as a security hedge, but that confidence began to slip...