 In the wake of the United Nations ( UN ) forces ' successful landing at Inchon and the subsequent destruction of the Korean People 's Army ( KPA ) by mid @-@ 1950 , the Eighth United States Army crossed the 38th parallel and advanced rapidly towards the Sino @-@ Korean border . Alarmed by this development , China 's Chairman Mao Zedong ordered the Chinese People 's Volunteer Army ( PVA ) to intervene in Korea and to launch the First Phase Campaign against the UN forces . Between October 25 and November 4 , 1950 , the PVA 13th Army surprised and defeated the Republic of Korea ( ROK ) II Corps and the US 1st Cavalry Division in a series of battles around Onjong and Unsan , destroying the right flank of the US Eighth Army while forcing the UN forces to retreat back to the Ch 'ongch 'on River . Although Chinese forces were able to break through the UN line , logistics difficulties forced the Chinese to withdraw on November 5 , 1950 . 
