Donald Trump reportedly revised the fiery anti-immigration speech he gave Wednesday night in Phoenix, Arizona, after Mexican President Enrique Peña Nieto tweeted that he’d told Trump his country would not pay for his proposed border wall.

During a joint press conference held after their private Wednesday meeting, Trump told reporters that he and Peña Nieto “didn’t discuss” his controversial proposal. But the Mexican president tweeted afterward, in Spanish, “At the start of the conversation with Donald Trump I made it clear that Mexico will not pay for the wall.”

Al inicio de la conversación con Donald Trump dejé claro que México no pagará por el muro. — Enrique Peña Nieto (@EPN) August 31, 2016

According to a report from the Wall Street Journal, when Trump saw Peña Nieto’s tweet, his first thought was of retaliation.

“I have to add back the line that Mexico will pay,” Trump reportedly told the people who were with him when he saw the tweet, arguing that he could not let it go unanswered.

Later that night, in a speech applauded by white nationalists and anti-immigrant groups, he suggested that Peña Nieto would have a change of heart. “They don’t know it yet,” he said, “but they’re going to pay for the wall.”

“I had no choice,” Trump told the Journal in an interview on Thursday regarding the revision, but said that he liked Peña Nieto “very much.”

For his part, Peña Nieto has maintained Mexico won’t be writing a check to fund Trump’s big, beautiful, possibly-metaphorical border wall.

This isn’t the first time that Trump has weaponized Twitter to respond to his political opposition. In response to Hillary Clinton’s charge that Trump is a man “you can bait with a tweet” during her speech at the Democratic National Convention, Trump fired back with a whole volley of attacks on Twitter.