Former Mexican President Vicente Fox said Friday that relations between the United States and Mexico have plummeted to a near-historic low since President Donald Trump took office last week.

“We are at the very lowest point since the war between Mexico and the United States,” Fox said on NBC’s “Today” show, referencing the nearly two-year Mexican-American War prompted by the U.S. annexing Texas in 1845.

Tensions came to a head this week when Trump signed an executive order to advance construction on a border wall between the two countries ― a move fiercely opposed by the Mexican government and many Democrats in Congress. The following day, Mexican President Enrique Peña Nieto canceled a trade meeting with Trump that had been set for next week.

Trump has brought back a very strong Mexican spirit. And we’re ready for the trade war. Former Mexican President Vicente Fox

Despite Trump’s long-standing pledge to make Mexico pay for the wall, Nieto has repeatedly insisted his country would never foot the bill. Trump said earlier this month that the American taxpayers will now fund the wall’s construction while his administration finds a way to make Mexico “reimburse” the U.S.

White House press secretary Sean Spicer on Thursday floated the concept of a 20 percent tax on all Mexican imports as a way to pay for wall, which drew sharp criticism from both Republicans and Democrats. Spicer walked backed the suggestion hours later, claiming the tax was simply a policy “option.”

Fox, who was Mexico’s president from 2000 to 2006, said Trump’s vow to build the wall has strengthened nationalism in his country.

“Trump has brought back a very strong Mexican spirt,” he said. “And we’re ready for the trade war. We’re ready, of course, for not paying for that wall.”

The former Mexican president has been an outspoken critic of Trump and his border wall proposal.

“I’m not going to pay for that fucking wall,” Fox told Fusion in February. “He should pay for it.”

He also frequently uses Twitter to denounce Trump.

Trump may ask whoever he wants, but still neither myself nor Mexico are going to pay for his racist monument.

Another promise he can't keep. — Vicente Fox Quesada (@VicenteFoxQue) January 6, 2017

go ahead, #trump. This will prove that the system made you win, not the people. Losing by 3+ million votes is not a mistake. https://t.co/0GWJvYcSgv — Vicente Fox Quesada (@VicenteFoxQue) January 25, 2017

The proposed wall is at odds with the growing reality of unlawful immigration. Immigration hardliners often claim huge numbers of undocumented people are crossing the border into the U.S., but the number of people doing so is at a 40-year low. Reports even show that more undocumented Mexican immigrants are leaving the U.S. than entering.

Still, Trump told ABC’s David Muir on Thursday that building a border wall is “very important.”

“We have to build the wall,” Trump said. “We have to stop drugs from pouring in. We have to stop people from just pouring into our country.”

Fox also appeared on MSNBC’s “Morning Joe” on Friday, and pleaded with Trump to “please grow up.”

“Now, you’re president of a great country, the most powerful nation in the world,” Fox said. “Be ― or try to be ― a real president.”