There is a crisis on America’s border with Mexico. The number of people arriving there has skyrocketed in the years since the pandemic, when crossings fell drastically. The scenes coming from the border, and from many US cities that have been touched by the migrant crisis, have helped elevate the issue in voters’ minds. But for all the attention the topic gets, it is also widely misunderstood. The last few decades have seen a series of surges at the border and political...