Following the capture of Chongju the US 21st Infantry Regiment had set off rapidly along the road to Sonchon to the west. Encountering only one strong KPA position which they quickly turned, by noon on 1 November the lead battalion had reached Chonggodong, just from the Yalu River where the Americans clashed with another KPA armoured force. To the north meanwhile, the US 5th and 9th Infantry Regiments of the US 24th Infantry Division secured Taechon and Kusong, before advancing to within of the Manchurian border. However, during the last weeks of October the Chinese had moved 18 PVA divisions 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 PVA 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 PVA 9th Army Group; in total 30 divisions composed of 380,000 men. The PVA 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. With the US 24th Infantry Division ordered back to the Chongchon River as a result, the 27th British Commonwealth Brigade also began moving south.