From the outbreak of the Korean War and the invasion of South Korea by the North , the North Korean People 's Army had enjoyed superiority in both manpower and equipment over both the Republic of Korea Army and the United Nations forces dispatched to South Korea to prevent it from collapsing . The North Korean strategy was to aggressively pursue UN and ROK forces on all avenues of approach south and to engage them aggressively , attacking from the front and initiating a double envelopment of both flanks of the unit , which allowed the North Koreans to surround and cut off the opposing force , which would then be forced to retreat in disarray , often leaving behind much of its equipment . From their initial June 25 offensive to fights in July and early August , the North Koreans used this strategy to effectively defeat any UN force and push it south . However , when the UN forces , under the Eighth United States Army , established the Pusan Perimeter in August , the UN troops held a continuous line along the peninsula which North Korean troops could not flank , and their advantages in numbers decreased daily as the superior UN logistical system brought in more troops and supplies to the UN army .