UNSAN
25th October - 4th November, 1950

After the Inchon-Seoul campaign the UN faced an immediate anti-climax as no adequate follow-up plan had been devised. For even though X Corps had provided the anvil, the hammer of the 8th Army had failed to fall swiftly upon the NKPA along the Naktong line. The NKPA commanders had wisely kept the truth from theirtroops, ordering a steady withdrawal north. Had the front line soldiers known they were trapped, the anticipated routmay well have developed. As it was, they fought as well as ever, delaying the 8th Army break-out. This bought enough time for substantial portions of the NKPA to flee north of the 38th parallel. The resultant situation forced a crucial decision upon the UN, which it had not properly anticipated. Should they pursue the NKPA across the 38th parallel or not? To cross the line might ensure the defeat of the NKPR and re-unify Korea, but only at the risk of prompting military intervention by Russia or the People's Republic of China. To stop at the border would allow the NKPA to rebuild, perhaps to launch another invasion.
In September 1950 the US National Security Council studied the matter and reported its findings to the Departments of State and Defence. They concluded that crossing the 38th parallel was not necessary for victory and that intervention by Russia and/or the PRoC was highly probable. If the line was to be crossed, three conditions were required. First, no UN troops were to cross the borders of Russia or the PRoC. Second, only ROK troops should be permitted to proceed north of the 38th parallel. Third, if either Russia or PRoC intervened, or threatened to do so, the operation must be cancelled.
The US Joint Chiefs of Staff dismissed the report on the basis that military doctrine required the destruction of the NKPA to prevent a renewal of the war. They believed the ROK troops could mop up, with other UN forces confined to the south. The Central Intelligence Agency concluded that since the PRoC had failed to intervene on behalf of the NKPR thus far, such action was no longer likely.
In October, the UN passed a resolution allowing ROK forces to cross the 38th parallel. From late September throughout October, the PRoC used various public forums to express its concern and declared its intent to assist the NKPR. No action was taken against the ROK forces, although on October 10th, the PRoC clearly warned it would enter the war if US troops crossed the line.
MacArthur knew the PRoC had massed troops along the Yalu but termed their intentions "unclear" and concluded no intervention was likely. The major stumbling block to UN thinking was that, while PRoC intervention in August would likely have sealed the fate of Pusan, and since the NKPR was nearly defeated by October, there was no longer reason for intervention.
On October 19th, the NSC relented and President Truman allowed MacArthur to enter the NKPR if militarily necessary.After MacArthur's victory at Inchon, no one dared oppose him again. Seeing the defeat of the NKPA as militarily necessary under the circumstances, on October 25th, MacArthur ordered the whole of Korea secured by UN troops. MacArthur had never agreed with Truman's policy and often publicly called for military opposition to communism, even at the risk of war with Russia and the PRoC. As a throwback to WWII reasoning, a "total" victory was now sought over the NKPR. The PRoC had its own interests to protect and had been satisfied to allow the NKPR to conduct its own war, particularly since their victory had appeared likely. Even when the tide turned the PRoC did not react. When ROK forces entered the North, the PRoC still restrained itself fearing no harm from Korean troops. When, however, MacArthur appeared to have been given free rein and was racing towards the Manchurian border, the vital portal to the PRoC was exposed to hostile forces. Worse yet, MacArthur was willing to defy his own President and had oft stated his desire to invade the PRoC and reinstate the Nationalists to power. It is no mere coincidence that the Chinese sprang into action the day following MacArthur's order sending US troops across the 38th parallel!
Ordered north with "all possible speed", the ROK II Corps rushed ahead. As part of an uncoordinated advance, the Corps was spread along the Chongchon river valley. The men of the Corps' 1st Division, perhaps the ROK's best troops, were unprepared to deal with the unknown danger ahead. Arriving at Unsan on October 25th, a company of US Army tanks led the 15th Regiment north of the town, until stopped cold by a road-block. Frustrated by this fierce new enemy, a few prisoners were rushed to General Paik for interrogation. Having fought similar troops in WWII, General Paik knew at once that these were soldiers of the People's Liberation Army of the PRoC.
Despite warnings from the ROK II Corps, MacArthur dismissed the presence of Chinese troops as mere "volunteers" and of no special significance. Even on the brink of disaster, UN commanders refused to believe the PRoC would intervene. More enemy troops massed from all sides. Huge forest fires spewed enough smoke to prohibit effective air support and artillery direction. Expert use of camouflage concealed an enemy who moved with swift silence along the hills, only to descend at night with a cacophony of whistles and bugles.
By the end of the month, the ROK II Corps had ceased to exist. 1st Cavalry troops were sent north to hold the line. The 8th Regiment arrived first, followed by the 5th Regiment. They too were encircled, with supplies cut and losses mounting. It was all they could do to flee south, leaving most of their equipment behind. Yet another debacle had been forced on the UN.
The Chinese Communist Forces, as they would be called throughout the war, had shown no fear of US firepower and a willingness to suffer heavy losses in order to defeat their foe. Yet having won their battles, they did not pursue the beaten UN forces. Still puzzled by the enigmatic Chinese, MacArthur conceded that this was the "beginning of a new war".