Less than one week into his presidency, Donald Trump has taken the first steps in making his vision of a massive barrier between the U.S. and Mexico a reality, signing an executive order Wednesday afternoon calling for the “immediate” construction of a sprawling border wall to separate the two nations.

Though the move is likely to appeal to his core supporters north of the border, one place where Trump’s efforts are not playing well is Mexico, despite the president’s dubious assertion Wednesday afternoon that “our relationship with Mexico is going to get better.”

By Thursday morning, less than 24 hours after his order was signed, Trump’s dubious optimism suffered a public blow when Mexican President Enrique Peña Nieto announced that he would not be attending a meeting in Washington, D.C., scheduled for next Tuesday.

Peña Nieto’s decision followed escalating tensions between his government and the newly empowered Trump administration that appear to have reached a critical point following Wednesday’s signing.

Throughout his campaign, Trump insisted the Mexican government could be forced into paying for the wall, which would likely cost billions of dollars. He has walked that back in recent weeks, asserting instead that Mexico would reimburse the U.S. for the project.

“We’ll be reimbursed at a later date from whatever transaction we make from Mexico,” Tump said in an interview with ABC News Wednesday. “I’m just telling you there will be a payment. It will be in a form, perhaps a complicated form.”

Peña Nieto has repeatedly denied Trump’s assertion. In a televised statement Wednesday night, the Mexican leader fired back at his counterpart in the White House. “I have said it over and over again: Mexico will not pay for any wall,” he said.

Responding to the Mexican president through his preferred medium of public communication, Trump tweeted Wednesday night that the meeting might be cancelled if Mexico refuses to pay for the wall. Peña Nieto beat Trump to the punch, however, saying in a tweet of his own the following morning that the Mexican government had informed the White House that he would not be attending the meeting.

“It is becoming a huge political issue,” Alejandro Hope, a national security expert and former intelligence analyst in Mexico City, told The Intercept.

As news of Trump’s executive orders began to break, “social media exploded” in Mexico, Hope said, with numerous prominent Mexican figures calling on President Peña Nieto to boycott his meeting with Trump.