Wubei Zhi (Vol. 86)

Yu Maoheng says: Shanxi faces the northern deserts and is protected by mountains and rivers. It was the capital region of the ancient sage-kings Yao, Shun, and Yu. It possesses a position that commands the Central Plains. North of Xin and Dai, the northern invaders frequently launch incursions. Because Datong is close to the invaders' dens, a military garrison was specifically established there. It provides mutual support with the Yansui and Xuanfu garrisons. The three passes of Yanmen, Piantou, and Ningwu are all fortified. Although the signal fires are constant, the invaders from the Ordos Loop are the most urgent threat.

Furthermore, in the areas under Taiyuan Prefecture, such as Xing, Kelan, Baode, and Hequ, the region is separated from the invaders by only one river. Whenever there is a major invasion, they enter the territory, cutting off communications and leaving the land desolate and withered. If defenses are even slightly withdrawn, the doors of the province are open to strong enemies. As for the miners of Weizhou, the garrison soldiers of Linjin, and the vagrants of Luzhou, they hide in mountain valleys, emerging periodically to plunder. The roads of Yize are long and remote, and the surrounding Taihang Mountains are deep and dense with forests where bandits emerge. If there were...

Records of Strategic Assessment, Assessment Nine, Geography Nine