Extract the answer to the question from the following context. Question: What partial definition was created by Nakdong? Context: By August, the KPA had pushed back the ROK Army and the Eighth United States Army to the vicinity of Pusan in southeast Korea. In their southward advance, the KPA purged the Republic of Korea's intelligentsia by killing civil servants and intellectuals. On 20 August, General MacArthur warned North Korean leader Kim Il-sung that he was responsible for the KPA's atrocities. By September, the UN Command controlled the Pusan perimeter, enclosing about 10% of Korea, in a line partially defined by the Nakdong River.
Answer:
Pusan perimeter