But Mr. Peña Nieto surprised Mr. Trump when he stated flatly that his country wouldn’t pay for the wall. Immediately, Mr. Giuliani responded for Mr. Trump, “That’s off the table.” With that, Mr. Trump moved to another topic.

A person close to Mr. Peña Nieto said that “since there was such a clear disagreement in preparatory conversations over issues about the wall and its payment, both parties agreed not to discuss them at the meeting, and instead talk about other topics, such as the great contribution that the Mexican community makes in the U.S., illegal drugs and weapons trafficking, bilateral trade, within North America and the rest of the world.” The person added: “Before the conversation began, Mr. Peña Neto reiterated to Mr. Trump that Mexico won’t pay for any wall, and as agreed, they discussed other topics.”…

Mr. Trump and his team were stunned, advisers said. News reports were questioning why Mr. Trump didn’t discuss the payment issue at the joint press conference.

“I have to add back the line that Mexico will pay,” Mr. Trump said, according to the people with him, adding that he couldn’t let that tweet go unanswered. Mr. Giuliani, a lawyer and onetime prosecutor, said he told Mr. Trump that the talking-points deal was effectively breached by the tweet, which gave Mr. Trump the right to talk about his insistence that Mexico would pay for the wall.