An Qingxu
In January 757 AD, An Lushan was killed by his own son An Qingxu (安慶緒). In the same month, An Qingxu ordered general Yi Ziqi (尹子奇) to besiege Suiyang. Yun Zi Qí joined with Yang Chu Zong before the siege, with a total army size of over 130,000 men. The governor of Suiyang at the time, Xu Yuan (許遠), knew of Zhang Xun 's fortress defense abilities and asked him for help. Zhang Xun knew that if Suiyang fell, the rest of Tang's territory south of the Yangtze River would be threatened. He immediately agreed to help and participated in the Battle of Suiyang.