The National Protection War broke out in 1915 when Yuan Shikai declared himself emperor and attempted to establish an Empire of China. Li Genyuan () sent a representative to Southeast Asia to invite Cen back to China to join the revolutionaries in their war against Yuan Shikai. On 4 January 1916, Cen returned to Shanghai, met Liang Qichao, and discussed with him how to deal with Yuan Shikai. He wrote letters to his former subordinates Lu Rongting and Long Jiguang, who were in control of Guangxi and Guangdong respectively, to declare independence from Yuan Shikai's regime. On 19 April 1916, Cen, along with Liang Qichao and Lu Rongting, met in Zhaoqing, Guangdong, where they established a National Protection Military Government. Cen was nominated to be commander-in-chief of the National Protection Army, with Liang Qichao as his chief of staff. Cen declared war against Yuan Shikai and proclaimed, "(...) If Yuan Shikai lives, I die; if Yuan Shikai dies, I live!"