The North Korean army opened the southward invasion simultaneously at every front line crossing the 38th parallel at June 25, 1950, 04:00. This southward invasion operation of the North Korean army was to occupy Busan within 50 days and spread communization in the whole Korean Peninsula.At that time, the military power of the North Korean army was of 3 independent regiments, 1 tank brigade, 5 defense brigades, 10 infantry divisions, etc, with 191,680 soldiers, 4,700 marines and 2,000 air men, summing up to 198,380 men as total military power.
In those days the military power of the ROK forces was of 96,140 soldiers, 7,715 marines and 1,897 air men in 2 independent regiments and 8 infantry division. The total military power was 105,752 men. Among these, the ROK forces placed 4 divisions and 1 brigade at the front line area to perform front line defense mission. The remaining units were arranged at the communications zone to execute the liquidation of the communist guerrillas and the warning mission. At that time, the preparation for operation of the ROK forces was not ready for regular warfare.
After the 38th parallel was attacked, Seoul, Chuncheon, and Gangneung fell on the 28th of June. At July 4, the Han river line broke through and the army was withdrawn to Pyeongtaek.At this time, Wonju, at the central-front line, fell on July 2 and at the eastern line stalling operations were proceeding as Samcheok has fallen at July 4. Meanwhile, the dispatch of U.N forces was settled and as the member of the UN forces, the leading troop of U.S 24th infantry division arrived at Pyeongtaek. The ROK forces joined the UN forces and formed the united force at the line connecting Pyeongtaek-Jecheon-Uljin at July 4.
At this point, the U.S ground army was in charge of the western front, and the ROK forces took charge of the middle west and the eastern front. While the ROK and the UN forces went over these stalling tactics, 90% of the South Korean territory was lost. In the meantime the UN military headquarters was established and the right of command of the ROK forces was handed over to the UN military headquarters. Also the UN forces acquired marine and air superiority satisfying the operation command system and the operation basis of the UN forces for execution of the Korean War. During that time the ROK forces restored their military power on the level of prewar period and accumulated the essential combat experience. Moreover, at this period, the naval and air forces from U.S, England, Australia, Canada and the naval forces from France, the Netherlands, New Zealand, arrived at the Korean operation zone and the scale of the U.S ground army has reinforced into 2 regiments and 3 divisions. On the other hand, the North Korean army was showing a rapid decline of military power as the result of loss of military power in their continuous offensive actions.
The North Korean army, having advanced to the Nakdong river line, opened a river-crossing operation from August 5 in their last, full force and ventured on the 2 final attacks to occupy up to Busan on August and September. However their succeeding back-up was not available and finally around September their military power reached the limit unable to continue further attacks.
On the other side, the ROK forces and the U.N force secured predominance by continuous supports and the arrival of reinforcements. From this period the Korean army and the U.N forces acquired the leadership of the operation. Therefore, from then, the North Korean army faced the strong resistance of attack and failed to take over Daegu. At September 15, after 82 days of opening the attack, their scheme was frustrated by the all-out counterattack of the U.N forces.