South Korea and the United States began a new round of war games Monday, involving about 30,000 U.S. forces and 56,000 South Korean personnel.
The 10-day operation is the second massive joint military exercise in less than a month. But unlike the earlier drill in the Sea of Japan, this exercise is land-based and involves relatively few troop movements.
Much of the training will take place on computer screens as commanders practice how they would respond to North Korean nuclear, missile and submarine attacks.
North Korea warned that its military would be "merciless" in its response to the joint exercises. Pyongyang issued a similar threat in response to anti-submarine exercises South Korea held last week off its west coast.
South Korean President Lee Myung-bak on Monday called the most recent drills "an exercise for peace and deterrence."
Mr. Lee used a Sunday speech marking the anniversary of the Korean peninsula's liberation from Japanese rule to outline what he called the path toward reunification.
The South Korean leader said it is time for North Korea to make what he called "a courageous change." He also warned that his government will not tolerate further "provocations" from the communist North.
In his reunification proposal, the South Korean president proposed several steps, including a "reunification tax" to offset the expense of merging with the impoverished North and a so-called "peace community" after the peninsula is cleared of nuclear weapons.
He said the next step would include measures to carry out inter-Korean exchanges for the development of the North Korean economy.
Some information for this report was provided by AP and AFP.