Human: What was the reasoning behind Wei Yan's Ziwu Valley Plan? Assistant: 
Wei Yan's reasoning for his Ziwu Valley Plan was recorded in the "Weilüe", which was then added as an annotation to his biography in the "Sanguozhi". It stated: "Wei Yan received intelligence that the defender of the strategic city Chang'an, Xiahou Mao, was cowardly and incompetent. Thus, he reasoned, it would be easy for him to take 5,000 troops (and another 5,000 to carry supplies) across the Qin Mountains