To the dismay of its North Korean neighbours, South Korea began a major naval exercise in the Yellow Sea on Thursday, the largest since 46 South Korean sailors died in March in the sinking of a warship. The five-day exercise involves about 4,500 personnel and all four branches of the military. Seoul, which oversaw an investigation into the March sinking of the Cheonan, claims a North Korean submarine sank the corvette and is demanding an apology. Pyongyang has vehemently denied the accusation. The drill irked North Korea, which promised “strong physical retaliation” against the drill, but experts say a naval attack on well-prepared South Korean forces is unlikely. The warship sinking heightened tensions between the two countries, who were at war from 1950 to 1953. Because the war ended in an armistice, not a peace treaty, the two nations are still technically at war.