The urgency of the threat to Pusan Perimeter prompted the US Marine Corps 1st Provisional Marine Brigade to be brought in to reinforce the US Army troops . In two weeks of heavy fighting , the US forces were able to force the North Koreans out of the Naktong Bulge region . The North Koreans were further repulsed after the UN counterattack at Inchon , which culminated in the virtual destruction of the North Korean army .