LAOAG, Philippines—Just off the windswept beach, a camouflage-painted unmanned vessel reminiscent of a Chinese amphibious fighting vehicle advanced through the sapphire waters of the South China Sea. Drones buzzed ahead as American, Filipino, and Japanese soldiers waited nearby. Then two rockets whooshed from HIMARS launchers hidden in the sand dunes, heading out to target simulated enemy warships in deep water. The battle had begun. This counte…