 During the last weeks of October the Chinese had moved 18 divisions of the People's Volunteer Army across the Yalu River under the overall command of Marshal Peng Dehuai in order to reinforce the remnants of the KPA.Undetected by US and South Korean intelligence, the 13th Army Group crossed the border on 16 October and penetrated up to into North Korea, and were reinforced in early November by 12 divisions from the 9th Army Group; in total 30 divisions composed of 380,000 men.The Chinese subsequently ambushed MacArthur's forces which were now widely dispersed, decimating ROK II Corps at Onjong and encircling and overrunning the US 8th Cavalry Regiment at Unsan.