Benjamin Netanyahu is spending US $75m to build a 30 km-long fence along Israel’s Jordanian border "to defend...against the wild beasts," referring to his Arab neighbors. Donald Trump is aiming for a vertical borderline with Mexico, saying of his 30 ft-tall wall, "I want it to be so beautiful...someday they'll call it the Trump Wall." Everyone knows walls are penetrable. Long before Berlin's toppled, the Greeks used a wooden horse to breach the ramparts of Troy. Barbarians bested Hadrian’s Wall, built by the Romans in northern England, and Genghis Khan’s ...