article: Donald Trump has fixed on immigration as one of the causes of the tide of anger on which his campaign has risen. If it's not the threat to American jobs from an influx of cheap skilled labour, it's the steady flow from Mexico, much of it illegal, that he says is unsustainable. Hence his famous promise to build a 2000-mile wall, which has come to define his campaign. He is right that this nation of immigrants struggles with the question, and his proposed "solution" has now turned it into a dividing line that will help to shape the autumn campaign. There is a sour whiff in America of the European immigration debate: never before has it been used so deliberately to raise the political temperature. Mr Trump knew what he was doing, and it has worked. Donald Trump wants to deport every single illegal immigrant - could he? Trump's foreign policy: So crazy it just might work? Trump's immigration plan savaged by rivals Why Mexicans are troubled by Donald Trump But visit the border in southern California - where they say it is the busiest international crossing point in the world - and you witness the complexities, not the simplicities. I stood looking through the fence with border agent Chris Harris, and he told me: "There's no silver bullet. No easy answer. A wall won't solve it." I spent a few hours with him on his stretch of the border, about 20 miles south of San Diego. He is an interesting witness - a Republican voter, who has worked all his life in law enforcement (he was a narcotics detective and then a prison officer in New York), the union representative for the border guards, who spends every day with his colleagues on the stretch of border running due east from the Pacific shore. When he arrived nearly 20 years ago, the border seemed hopelessly porous, with untold thousands of illegal immigrants crossing easily, slipping through the undergrowth, mounting elaborate deceptions to occupy the guards and bring whole parties over, squeezing through tunnels. That has changed. Chris Harris still reckons that 5,000 (and probably more) get through each year on the short six-mile stretch where we're walking - but compared with the past, that is a trickle. Across the rough terrain, roads have been built for patrols, navigable canyons blocked, and two fences built. They are dotted with repairs where illegal immigrants have cut through, and we handled one of the metal ladders designed to be hooked over the top and get a few people over in a few seconds in the dark. It is a cat-and-mouse game, constant and dangerous. And where many of the crossings in the past were by individuals desperate to find work, much of the traffic is now controlled by violent people-smuggling cartels who are running a big, nasty business. The drama on the border can easily be used to make it sound like a simple problem of law enforcement, and right and wrong. But there are problems apart from the fact that the agents believe a
summary: The Mexican border is a line that sends an electric charge through the US presidential campaign.