A large bearded man named Tommy rolls a shopping cart full of wooden crosses into a small square off the Pan American Avenue. Someone has painted them all white. One block south of where we stand, the United States ends abruptly. Between a double wall made of iron, a concrete trench is filled with loose coils of concertina wire. The metal teeth glint under the red sun. To our west sits an air-conditioned McDonald’s and just past that a Wal-Mart sprawls into the horizon. The people of Douglas and a few Mexicans from Agua Prieta — the very few affluent enough to secure a nonimmigrant visa — walk the broad aisles of the big-box store making their purchases...