IN 1971 THE barrier that separated San Diego from Tijuana was a bit of chicken wire. Even that was too much for Pat Nixon, who was first lady at the time. When she visited the beach at the very south-western edge of California she decided she wanted to meet the Mexicans queuing to greet her. So she asked the mayor of Tijuana to tear down the fence. “I do hope that this will be a common beach”, she said. “Because we’re such good friends with Mexico, I don’t think we need a border.”