Breaking The Perimeter! By August 1950, the North Korean Army had overrun most of the Korean peninsula, and was advancing rapidly toward Pusan, the last open port in South Korea. The US Army hastily formed a perimeter along the arcing Naktong River to stop - or at least delay - the final North Korean breakthrough. Every available US unit in Korea, including a Marine brigade, was thrown into the fight against three North Korean infantry divisions converging to crush what became known as the "Naktong Bulge."