China’s seizure of an American underwater drone in the South China Sea this month was a reflection of Beijing’s concerns about the growing use of such vehicles in the contested waters and would spur efforts to build up its own military capabilities beneath the waves, military experts say. Beijing played down the seizure of the unmanned underwater vehicle, which the United States said was operating lawfully in international waters about 50 nautical miles northwest of Subic Bay in the Philippines when it was taken by a Chinese navy ship on December 15, and returned the drone five days later to a US navy ship close to where it had been seized.