Fortified frontiers

Ditches, barbed wire fences, brick and stone walls are all means of building obstacles between two peoples, sometimes to keep one group in and sometimes to keep the other group out. The Great Wall of China, the Maginot Line, and the Berlin Wall are examples.

The USA is currently digging a ditch 1.5 metres deep and 4 metres wide near the main border crossing with Mexico at Tijuana to "address a security and safety concern".