Prince of Qin
In spring 621, with Emperor Gaozu's son Li Shimin the Prince of Qin putting the Zheng capital Luoyang under siege, Du sent his generals Chen Zhengtong () and Xu Shaozong (), with 2,000 men, to assist Li Shimin in his campaign. Chen and Xu were able to capture the Zheng city Liangcheng (梁城, in modern Ruzhou, Henan).