The Pentagon said on Friday it had issued a formal protest to Beijing demanding the return of an underwater drone seized by a Chinese warship in the South China Sea, an incident that risked increasing tensions in a region already fraught with great-power rivalries. A Defense Department official said that the unmanned underwater vehicle was discovered missing on Thursday when the crew of the United States Navy vessel Bowditch tried to retrieve it. Whatever the case, the Pentagon said that China had no right to seize the drone.