Defenses by Tom Sharp

I asked an official at a mountain pass why soldiers were building ramparts there. He showed where they’d built a narrow passage that one soldier with a sword could defend against all comers, but I asked why any rebel would enter it. Even a wall as high as the mountains wouldn’t hold back an army. Like birds, they’ll just fly around it, come around behind you, and your deaths won’t help defend the distant capital.

潼关吏 by 杜甫 (Du Fu)

士卒何草草，筑城潼关道。大城铁不如，小城万丈余。借问潼关吏，修关还备胡？要我下马行，为我指山隅。连云列战格，飞鸟不能逾。胡来但自守，岂复忧西都？丈人视要处，窄狭容单车。艰难奋长戟，万古用一夫。哀哉桃林战，百万化为鱼。请嘱防关将，慎勿学哥舒。