First came the concrete markers engraved in multiple languages. Naval aviators from the Philippines would spot them during surveillance flights in the mid-1990s and dispatch forces to remove them. Then came the huts—small, wooden structures teetering on stilts on uninhabited islands, fit maybe for fishermen to take shelter during storms. They looked innocuous enough, one of the pilots, Alberto Carlos, recalls thinking. Only later did Carlos understand that he was witnessing the initial phases of China’s conquest of the South China Sea. On rocky, barren islands, Beijing installed intelligence-gathering equipment, long-range surface-to-air missile systems, and stealth fighter jets. Over the past decade, China has added more than 3,200 acres of land to its seven occupied outposts in the Spratly Islands, according to the U.S. Department of Defense. The South China Sea is perhaps the most contested waterway in the world. China, Vietnam, Malaysia, Brunei, the Philippines, and Taiwan all have competing claims there. But no actor has pursued those claims as belligerently as China. The Philippines complains that Chinese forces menace its sailors and fishermen …