中国人民解放军第四野战军
On 29 November 1948, the People's Liberation Army launched an assault on Zhangjiakou. Fu Zuoyi immediately ordered the Nationalist 35th Army in Beiping and the 104th Army in Huailai to reinforce the city. On 2 December, the PLA Second Field Army began to approach Zhuolu. The PLA Fourth Field Army captured Miyun on 5 December and advanced toward Huailai. Meanwhile, the Second Field Army advanced to the south of Zhuolu. As Beiping was at risk of being encircled, Fu recalled both the 35th Army and the 104th Army from Zhangjiakou to return and support the defense of Beiping before being "surrounded and destroyed" by the PLA.