The Pentagon demanded that China immediately return a U.S. Navy underwater drone that was captured in the South China Sea, in a confrontation certain to exacerbate tensions in a region where the government in Beijing has sought to assert greater control. China “unlawfully seized” the unmanned underwater vehicle Thursday while it was being recovered by a U.S. Navy oceanographic survey ship during routine operations 50 nautical miles northwest of Subic Bay in the Philippines, according to a Defense Department statement.