Chiang Kai-shek's clique began to focus on attacking northern Shaanxi. In December 1946, Hu Zong invaded the Guanzhong Division from the south.Zhang Zongxun, Xi Zhongxun and others led the Shaanxi-Gansu-Ningxia Field Army and the masses to actively prepare for the war.In February 1947, during the Battle of Xihuachi, the troops fought fiercely with the Kuomintang army and suffered heavy losses.After the Battle of Xihuachi, Peng Dehuai ordered Zhang Zongxun, Xi Zhongxun and others to lead the Shaanxi-Gansu-Ningxia Field Army to move closer to Yan'an and defend Yan'an.On March 13, 1947, Hu Zong began to attack Yan'an from the south, and the Yan'an defense battle began.On March 18, Chairman Mao, Zhou Enlai and others led the central agencies to withdraw from Yan'an.Immediately, the Northwest Field Corps was formed.