President Trump has not made it clear how he will collect the money. The idea he floated of a 20 percent tax on goods coming from Mexico fell like a ton of bricks when it was pointed out that American consumers would be the ones paying. In the campaign, Mr. Trump released a memo saying the United States could force Mexico to pay by blocking remittances, the nearly $25 billion sent home by Mexicans every year, money earned from hard labor that supports the poorest communities south of the border. It has also been suggested he will try to tax those transfers. Either action would be challenged in the courts, especially as most of that money has already been taxed, and migrants would look for alternative ways to move it. But he might succeed in getting some of his funds this way.

A note in the executive order to build the wall may offer a clue to another fund-raising tactic. Section 9 of the order demands that all the aid that has been given to Mexico over the past five years — probably around $320 million a year — be tallied up in a report for the president. Perhaps Mr. Trump is thinking of canceling future aid, or perhaps he’s going to claim that the money sent in the past has to be paid back to finance his “beautiful wall.”

If Mr. Trump runs up the national debt, he will be under increasing pressure to get back his wall dollars. And the way the wall is financed now could determine how the shakedown plays out. If Congress passes a bill to fund the wall, lawmakers need to force out any wording that commits to collecting the money from Mexico.

The demand that Mexico pay for the wall was the breaking point in other negotiations on trade and immigration. It was the issue that forced President Enrique Peña Nieto into a corner, so he had no choice but to cancel his meeting in Washington. Money aside, it became a question of national humiliation.

But perhaps this idea of humiliating Mexico is what really appeals to President Trump and his most fervent supporters. As María Eugenia Valdés, a political scientist at Mexico’s Autonomous Metropolitan University, said to me: “He wants to make an example of Mexico to show how he will deal with countries around the world.”

Shakedowns are cemented through violence. Behind Mr. Trump’s rhetoric, many Mexicans sense the implicit threat of American force, backed by a history of occupations and wars. “There is a real risk of the conflict turning into violence,” Ms. Valdés said. “This man is capable of anything.”

I hope such fears are crazy. They should be.