A newly built section of the U.S.-Mexico border wall at Sunland Park opposite the Mexican border city of Ciudad Juarez, November 9, 2016. (Jose Luis Gonzalez/Reuters)

IT WAS always far-fetched, bordering on laughable, that President-elect Donald Trump’s wall would be paid for by Mexico, whose leaders’ reaction to the proposition tended to be unprintable. Now Mr. Trump himself has acknowledged that for the time being, American taxpayers, not Mexicans, will be opening their wallets to pay for his “Great Wall.” Mexico, Mr. Trump insists, will reimburse the United States in, well, the fullness of time.

Much of Mr. Trump’s campaign bombast played the electorate for suckers, but the idea that Mexico would fork over billions of dollars for a wall was preposterous on its face. As a rhetorical device at his rallies, he delighted in using the wall as a call-and-response punch line: Who would pay for the wall? Mexico! The practicalities of making it happen, at a cost that could easily exceed $20 billion or $25 billion, are dizzying.

The president-elect, in typically pugnacious fashion, blames the “dishonest media” for not reporting what he says he really meant all along, which is that “any money spent building the Great Wall (for sake of speed) will be paid back by Mexico later!” Mr. Trump somehow forgot to mention that during the campaign.

The next step is inserting billions of dollars for the wall into a congressional spending bill that must be passed by April in order to keep the government running. Republicans in the House are preparing the ground for that, at Mr. Trump’s behest, deficits be damned. The Senate, where Democrats have the numbers to block legislation, may be a greater hurdle.

Prepare for a game of chicken between the two parties over which one would be to blame for a threatened government shutdown caused by arguing over a wall that Democrats oppose, a majority of Americans say they don’t want and Republicans have justified on grounds that the United States would not bear the cost. Will GOP deficit hawks give their blessing to the billions for a wall, on top of whatever additional billions the Trump administration seeks for its ambitious infrastructure program, or to repeal Obamacare?

It’s a fool’s game to expect a candidate to fulfill every campaign promise, but a Mexican-funded border wall wasn’t just any promise; it was Mr. Trump’s signature, repeated ad nauseam for the delight of jazzed-up crowds of gullible supporters.

Could Mr. Trump somehow force Mexico to pay for some portion of the wall? Perhaps he could frame it that way, after compelling a renegotiated trade deal on terms he declares tilted in America’s favor, or by attempting to seize remittances sent across the border back to Mexico, as he has threatened. Neither option looks probable, easy or quick — or advisable.

Washington has leverage. But it’s foolish to suppose that Mexico could not retaliate against the United States by imposing painful tariffs, fees, visa problems or impediments to the dizzying variety of bilateral and cross-border issues whose success relies on Mexican cooperation. It’s hard to fathom how embittering Mexico, by squeezing it for cash for a vastly wealthier America, would redound to the benefit of the United States.