Twenty yards north of the U.S.-Mexico border the steel fence forms a tall, cage-like enclosure around the plaza of San Diego’s International Friendship Park. A pair of armed Border Control agents and a towering metal pole bristling with surveillance cameras keep tabs on visitors to the American side’s flat, featureless expanse of paved concrete. The Mexican side is filled with food trucks, picnic tables, planters full of flowers, and people enjoying a Sunday afternoon. Friendship Park was dedicated by First Lady Pat Nixon in 1971 as a gesture of goodwill between nations. At first there was no fence; later, only a chain link. In 2009, citing concerns about the passing of drugs, weapons, and other contraband, the United States government built a secondary barrier before closing the park, at the time indefinitely. The area reopened in 2012, but with both visitation hours and the number of visitors under tighter control. A sign at the entrance now sets a 25-person limit on the number of people that can be inside the plaza, which is only open on Saturdays and Sundays between 10:00 AM and 2:00 PM. Saul Garcia and his cousin, Claudia Burrola, visit family members who aren’t allow...