By August, the KPA steadily pushed back the ROK and the Eighth United States Army southwards. Facing a veteran and well-led KPA force, and lacking sufficient anti-tank weapons, artillery or armor, the Americans retreated and the KPA advanced down the Korean Peninsula. During their advance, the KPA purged South Korea's intelligentsia by killing civil servants and intellectuals. By September, UN forces were hemmed into a small corner of southeast Korea, near Pusan. This 230-kilometre (140-mile) perimeter enclosed about 10% of Korea, in a line partially defined by the Naktong River.