South Korea and the United States on Monday launched a joint exercise aimed at enhancing defense readiness against potential security threats here, military officials said. The computer-assisted simulation exercise, dubbed Ulchi Freedom Guardian (UFG), will mobilize some 56,000 South Korean troops and 30,000 U.S. forces. "Ulchi Freedom Guardian is a key exercise in strengthening the readiness of Republic of Korea and U.S. forces," Gen. James D. Thurman, commander of the U.S. forces stationed here, said in a statement. "It is based on realistic scenarios and enables us to train on our essential tasks with a 'whole of government' approach," he added.