After the end of the Nationalist-Communist alliance in 1927 Xu went underground. He did not participate in the failed Nanchang Uprising, but led the failed Guangzhou Uprising shortly after. He did not become an associate of Mao Zedong, instead becoming one of the main military commanders of a rival Communist leader, Zhang Guotao. The army Xu commanded under Zhao was called the "4th Front Army". He served as Zhang's principle commander with Ye Jianying as Chief of Staff. During this time, he helped Zhang to establish new communist bases and expanded the 4th Front Army of the Chinese Red Army, despite the fact that his wife was executed by Zhang Guotao in his political purges. While under suspicion and the surveillance of Zhang's political commissars, Xu Xianqian lead the 80,000 strong 4th Front Army of the Chinese Red Army in Sichuan to victory against local warlords troops that numbered more than 300,000. Over 100,000 warlord troops were killed in conflicts with Xu's forces, and the remaining 200,000 deserted or retreated to other Nationalist-aligned areas.