Of course, equipment like cameras, motion sensors, drones, helicopters, and vehicles allow agents to watch long sections of the border. But the necessity of monitoring border walls points to one of the fundamental truths about them: historically, most have proved to be pretty useless.
当然，摄像机、运动传感器、无人机、直升机和汽车使得边防人员能够监控相当长的边境线。但监控边境墙的必要性直指一个关于它们的基本事实：从历史上看，它们大多毫无用处。最著名的是中国的长城，建成不过几十年就被穿越。二战中德国入侵法国时也轻松绕过了马奇诺防线。柏林墙也在建成后不出30年倒塌。