Mr. Trump’s anti-immigrant talk worked well on the campaign trail, as he convinced struggling Americans that foreigners were to blame for lost jobs and blighted communities. To carry out his promise of ramped-up immigration enforcement and border security, he will need to convince Congress and American taxpayers that spending billions to execute his plan is a worthy investment.

As is so often the case with Mr. Trump, the facts are not on his side. Illegal immigration to the United States has been on a downward trend in recent years, even as spending on border security has soared. Between 1983 and 2006, an average of 1.2 million people a year were apprehended trying to enter the country unlawfully. In 2016, just over 415,000 were caught trying to enter; most were Central Americans fleeing violence and poverty. Meanwhile, border security funding has increased from $263 million in 1990 to $3.8 billion in 2015.

Mr. Trump would add significantly to that spending. He has insisted that Mexico will ultimately pay for the wall — a vow that is either deceitful or delusional. Mr. Trump ordered federal agencies to tally the foreign aid Mexico receives from the United States, which seems like a threat to withhold future assistance for initiatives such as narcotics enforcement and judicial programs.

Even if Mr. Trump was to cut off aid to Mexico, the savings would be modest; it got roughly $142 million in 2016, which doesn’t begin to pay for a wall along the 1,989-mile border. Besides costing billions, the type of barrier Mr. Trump has proposed would cause severe environmental damage and lead to lawsuits over private land.