The invasion was almost successful in conquering all of South Korea until the UN intervened, sending ground forces into the country under the command of the United States.

By October 1950, the KPA was effectively destroyed by the UN forces after the landing at Inchon, the breakout from the Pusan Perimeter and the UN September 1950 counteroffensive.

[4] In order to stabilize the rapidly collapsing Korean front and to push back the advancing UN forces, Mao authorized the First Phase Campaign, a bridgehead-building operation with the aim of destroying the ROK II Corps,[5] the vanguard and the right flank of the US Eighth Army, advancing up along the Taebaek Mountains in the middle of the peninsula.

[7] Onjong is a crossroad village located at the lower Ch'ongch'on River Valley, 10 mi (16 km) northeast of Unsan.

[9] On 24 October the ROK 6th Infantry Division of II Corps advanced westward from Huich'on,[2] and Onjong was captured on the same day.

[9] As a result, the ROK 7th Infantry Regiment managed to wander into enemy territory without much opposition, completely oblivious to the new threats surrounding them.

[8] The 3rd Battalion dismounted from their vehicles to disperse what they thought would be a small force of KPA,[8] but the two PVA regiments on the high ground immediately began pouring heavy fire onto the ROK left, front and right flanks.

[15] Although Colonel Ham Byung Sun, commander of the ROK 2nd Infantry Regiment, managed to rally his troops 5 km (3.1 mi) east of Onjong, the PVA were still able to penetrate the new position within an hour.

[25] About 875 officers and 3,552 other soldiers managed to escape, while Major Harry Fleming of the Korean Military Advisory Group was wounded fifteen times and was later captured by the PVA.

[28] This meant the right flank of the US Eighth Army was completely open to the PVA, which were now advancing south to overwhelm the UN forces.

[30] Only the stubborn defense of Kunu-ri by the US 5th Regimental Combat Team and the ROK 7th Infantry Division on 4 November managed to stop the PVA advance and prevented a disastrous defeat for the Eighth Army.

[35] During the planning of the PVA Second Phase Offensive, Peng would again focus his attention towards ROK II Corps at the Eighth Army's right flank,[35] resulting in a disastrous defeat for the UN forces at the Battle of the Ch'ongch'on River.