Even before Donald Trump landed in Mexico Wednesday, he found himself in a Twitter war with former Mexican President Vicente Fox – who went on a tear declaring the Republican nominee “not welcome” in the country.

Fox, an outspoken critic of Trump, started the fight when he said on CNN that he disagreed with current President Enrique Pena Nieto’s decision to invite the billionaire to the country to meet with him.

“He is not welcome to Mexico,” Fox told CNN. “We don’t like him. We don’t want him. We reject his visit.”

"I think President Peña is taking an enormous political risk by hosting Trump,” he said. “He will even be considered like a traitor because we don't accept to be offended like we just heard, and it should not happen that he comes here with a smile on his face. I think this is a big mistake on the part of President Peña."

Trump, as is often his way, fired back on Twitter.

“Former President Vicente Fox, who is railing against my visit to Mexico today, also invited me when he apologized for using the ‘f bomb,’” he said, referring to when Fox said he wasn't going to pay for "that f-----g wall."

This triggered a fiery response from Fox, also on Twitter, as he called on Trump to apologize and “stop lying.”