Tang forces led by Li Shimin defeated and captured Dou Jiande at the Battle of Hulao in the civil war that followed the collapse of the Sui dynasty. Battle of Hulao: The Battle of Hulao, or Battle of Sishui, was a decisive Tang victory over the rival Zheng and Hebei-based Xia polities during the transition from Sui to Tang. The battle took place during the Luoyang–Hulao campaign on 28 May 621 when a Xia army – led by Dou Jiande, ruler of Xia – was defeated attacking a smaller Tang army – led by Prince Li Shimin – entrenched at the strategic Hulao Pass. May 28, 621. Wiki link