Hangu Pass
From around the beginning of the 3rd century, the importance of the Hangu Pass was reduced, with the major fortifications and military bases moved upriver to Tongguan. In AD 923, the desperate Later Liang general Duan Ning again broke the dikes, flooding in a failed attempt to protect his realm's capital from the Later Tang. A similar proposal from the Song engineer Li Chun concerning flooding the lower reaches of the river to protect the central plains from the Khitai was overruled in 1020: the Chanyuan Treaty between the two states had expressly forbidden the Song from establishing new moats or changing river courses.