Which won’t stop the flow of money out of Washington. One example: The Homeland Security inspector general reported in 2014 that the Border Patrol had spent $360 million over eight years on drones but found “little or no evidence” that the drones made the border more secure.

Although facts are of little interest to this White House, all this budget-busting border mania is essentially for nothing. Illegal immigration from Mexico has trailed off in the last decade. And according to the Pew Hispanic Center, the net flow across the border is now less than zero.

Wait, there’s more. All the people Mr. Kelly rounds up will have to be detained and deported at taxpayer expense. Congress requires the Homeland Security Department to maintain about 34,000 immigration detention beds, at an estimated annual cost of $2 billion, or $5.5 million a day. Adding thousands more cells and beds will surely send that bill — like the profits of the private-prison contractors who have been cashing in on all this misery — through the roof.

Now let’s examine the cost to the economy.

If you do back-of-the-envelope calculations, you’re gonna need a big envelope. The American Action Forum last year estimated that expelling all unauthorized immigrants, and keeping them out, would cost $400 billion to $600 billion, and reduce the gross domestic product by $1 trillion.

Mr. Trump describes immigrants as rapist-murderer-terrorists, but what they really are is a pillar of the American economy, producing a net benefit of about $50 billion since 1990. Farms and restaurants, hotels, manufacturers, retail businesses — all sectors of the economy benefit directly or indirectly from immigrant labor.

As for taxes, unauthorized immigrants pay them, and if they work off the books, they pay into the system without taking out. They don’t collect Social Security and don’t qualify for food stamps or other benefit programs. They pay sales and property taxes, and since they are generally younger and healthier than the native-born population, they strengthen the safety net. The Social Security Administration estimates that unauthorized immigrations pay about $13 billion a year into Social Security and get only about $1 billion back.

It’s ridiculous to have to explain this to the president, but: If you take a population of 11 million people out of the country, or force them deeper underground, the economy will not be healthier. If you have to find foster parents for millions of abandoned American children of deported immigrants, society will not be stronger. Shrink our immigrant-rich economic sectors, send all that entrepreneurial energy to Canada and Mexico — America will not be better off.