How did the enemy exploit sustainment weaknesses to contribute to the US withdrawal to the Pusan Perimeter?

North Koreans knew that the American and South Korean forces where not ready to actually be in a large scale war  and they would be reliant on secure supply channels and infrastructure to continue fighting. The North Korean army had mastered the ability to out maneuver its enemies in guerilla war fair in order to envelope its enemies. American forces where not prepared to face this type of war as they where accustomed to focusing on a frontline. US forces that where established in time to fight where quickly cut off from their supply lines and forced to retreat.