President Trump stands behind his campaign promise to build a wall along the entire U.S.-Mexican border. On the campaign trail, he insisted he could get the job done for no more than $10 billion because he is the master of completing construction projects quickly and under budget. This past weekend the president took to Twitter to lash out at reports that the true cost of the border wall would be well north of $10 billion.

The critics are almost certainly correct. Mr. Trump fails to take into account the major hurdle the wall...