Severing the ties of an internationally integrated economy is a messy business. You cannot stop the world and get off. Walling yourself in is a good way of discovering just how many ways you depend on the outside. …. Border regions are naturally entangled. The only borders that are not hybrid melting-pots are those marked by oceans — a mere sea is not enough, as the Mediterranean illustrates — or by impassable desert, or by their human equivalent, something like the death strip of the Iron Curtain. Much as some Trumpists may long for a death strip, they will crave in vain. ….