The Ulchi Focus Lens command post exercise had been postponed as it was to coincide with an upcoming summit between the North and South Korean leaders, but Seoul decided to press ahead with the exercises when the summit was put off, despite objections from Pyongyang.
The joint exercise involving 10,000 American troops, will run until August 31, and will be based on computer simulations, with few field activities. Pyongyang has called the exercise a preparation to invade, but the U.S. and the South say it is purely defensive.
The second-ever summit between North and South Korea had been scheduled for August 28-30, but was postponed to October 2-4 due to catastrophic flooding in North Korea.
South Korea delayed the drills, scheduled to overlap with the meeting, after Pyongyang warned that the exercises would have a disastrous effect on ongoing six-nation talks to resolve North's nuclear problem.
About 28,000 U.S. troops have been stationed in South Korea since the end of the 1950-53 Korean War, which ended in a cease-fire, not a peace treaty, leaving the two Koreas formally at war.
