Oh , so the delicious irony. For many years I’ve been writing about the southern US border and the many ways in which Mexico has used it as a “steam valve” to get rid of people in perpetual poverty, as well as malcontents, violent criminals and political revolutionaries. Who could have foreseen a time when the conundrum would be reversed and Mexico would be crushed by an avalanche of its own unwanted citizens? But weren’t we told that migrants are an “economic boon” to any country lucky enough to have them? The argument among progressives and open border activists has...