What happens when two populists from neighboring countries fight for their respective constituencies? They don’t get along well, that’s what. President Donald Trump was scheduled to meet with Mexican President Enrique Peña Nieto next Tuesday, but the latter canceled the meeting after the two sparred over Trump’s order to complete the 2,000 miles of border wall authorized in 2006. A wall supported by Senate Majority Leader Chuck Schumer, as well as then-Senators Barack Obama, Joe Biden and Hillary Clinton.

Trump reiterated Wednesday, “There will be a payment [from Mexico]; it will be in a form, perhaps a complicated form.” (That might be some kind of border-adjustment tax on imports, as the White House floated Thursday.) Peña Nieto was having none of it, retorting, “I have said time and again, Mexico will not pay for any wall.” Which was a far friendlier way of saying what his foul-mouthed predecessor, Vicente Fox, had declared more than once: “Mexico is not going to pay for that f—king wall.”

Aside from pure Mexican pride (or, conversely, Trump’s humiliating demand), our neighbor’s leaders reject the wall largely because Mexico benefits financially from immigrants who cross our borders and send money back home to their families. Trump wants them to pay for the wall because they’re not doing much to stop the flow of immigrants — not only Mexicans fleeing that country’s horrendous drug war, but Central Americans passing through. Our southern border is a porous terrorist threat, too.

“I regret and reject the decision of the U.S. to build the wall,” Peña Nieto said, adding that it was divisive. Has he never heard that good fences make good neighbors?

In any case, Trump told his counterpart (via Twitter, of course), “If Mexico is unwilling to pay for the badly needed wall, then it would be better to cancel the upcoming meeting.” Thus Peña Nieto had little choice but to announce (via Twitter, no less), “We have informed the White House that I will not attend the working meeting planned for next Tuesday with @POTUS.” What else could he do? Both men are playing to their constituent citizens. Trump is obviously less popular in Mexico than he is here, so Peña Nieto had almost no reason to cave on the meeting. And Trump won election largely by way of his promises to secure the border and to not take crap from other nations. We expect both men are, at the moment, testing each other and setting their opening proposals for a deal.

(Updated.)