Korean War
Against the rested and re-armed Pusan Perimeter defenders and their reinforcements, the KPA were undermanned and poorly supplied; unlike the UN Command, they lacked naval and air support. To relieve the Pusan Perimeter, General MacArthur recommended an amphibious landing at Inchon (now known as Incheon), near Seoul and well over 100 miles (160 km) behind the KPA lines. On 6 July, he ordered Major General Hobart R. Gay, Commander, 1st Cavalry Division, to plan the division's amphibious landing at Incheon; on 1214 July, the 1st Cavalry Division embarked from Yokohama, Japan to reinforce the 24th Infantry Division inside the Pusan Perimeter.

Q: Why were KPA forces unable to adequately defend themselves against UN forces?
the KPA were undermanned and poorly supplied