On Monday, the US and South Korea held their second joint military exercise in a month. The scale of the drill outstripped that of the first drill, held late last month, by three times. Despite both Chinese and North Korean threats, the US and South Korean insistence on the drills was a response to North Korea’s alleged sinking of the South Korean Cheonan warship. It was also a reaction to China’s recent claim that the Yellow Sea and the South China Sea are part of its core interests.