Trump supporters understand the great fairness of his proposal. For years the US subtly encouraged illegal immigration to develop a whole underclass of legally vulnerable people to work in rich people's homes, as farm laborers, and other tough jobs for wages that only the vulnerable would accept.  With automation causing so many job losses of the kind that Mexicans can only wish they might have had,  these people are now more vulnerable politically. So the US is to build a political symbol of limited practical consequence that hopes to stem the tide of the disadvantaged even as the US creates vastly more of the disadvantaged. That Mexico should be asked to pay for this is only logical. Why can't those Mexicans understand? The former Canadian ambassador to Washington pointed out that Mexico has been doing so well that last year more Mexicans returned to Mexico from the US than entered the US from Mexico. Mr. Trump knows how to fix that.