In a meeting with U.S. Defense Secretary James Mattis in June, Chinese President Xi Jinping vowed that Bejing “cannot lose even one inch of the territory” in the South China Sea. That’s about as plain a warning as one gets: China will defend, militarily, it’s outsized claims in a body of water through which one-third of all global trade passes. The U.S. would have little difficulty in taking out militarily significant targets on any of the islands in question. But that’s not really the point. Through the construction of these artificial islands-turned-military bases, China acted brazenly but in a calculated...