A cave complex blasted out of the rocky coastline on China's southern island province of Hainan is home to one of the newest and potentially most lethal weapons in Beijing's arsenal: a home-grown submarine designed to launch nuclear-armed ballistic missiles.

So when the USNS Impeccable, a U.S. surveillance ship, was snooping in the area last month, China set a trap. Five Chinese vessels crowded around the U.S. ship. Crew members hurled chunks of wood into the Impeccable's path and used poles to try to snare its acoustic equipment....