 While the Eighth Army was advancing , the PVA 13th Army was hiding in the mountains with the 50th and 66th Corps to the west , the 39th and the 40th Corps in the center and the 38th and 42nd Corps to the east . Anticipating the UN advances , the Chinese planned a series of counterattacks to catch the Eighth Army off guard . Hoping to repeat the success of the earlier First Phase Campaign , the 38th and 42nd Corps would first attack the ROK II Corps and to destroy the UN right flank , then cut behind the UN lines . At the same time , the 39th and 40th Corps would hold the US IX Corps in place while preventing any reinforcements to the ROK II Corps . The 50th and 66th Corps would stay behind and check the advances of the US I Corps . 
