《战国策·燕一·苏秦将为从北说燕文侯》
况且秦国攻打燕国，要越过云中和九原，经过代郡和上谷，长途跋涉几千里，即使能够攻下燕国的城池，也根本没有办法占领它。
且夫秦之攻燕也，逾云中、九原，过代、上谷，弥地踵道数千里，虽得燕城，秦计固不能守也。
Moreover, if Qin were to attack Yan, it would have to cross Yunzhong and Jiuyuan, pass through Daijun and Shanggu, and travel thousands of miles; even if it were able to capture Yan's cities, there would be no way to occupy them.