The most difficult moment of Canada's NAFTA negotiations with the United States came when U.S. President Donald Trump announced he had struck a bilateral deal with Mexico, says Canada's ambassador in Washington

"The most difficult one for us was when the U.S. and the Mexicans came to a two-way deal on the NAFTA negotiations," David MacNaughton told CBC News.

"What ended up happening was not what we agreed to in advance," he said.

"The Mexicans and the Americans were going to be talking but only [about] bilateral issues. [But] they had involved a whole series of things that we agreed in advance were not going to be dealt with until we were back at the table."

Trump made the announcement in the Oval Office in late August, 2018, saying the deal would replace the existing NAFTA agreement.

Would Canada can be part of it? "We will see," Trump said at the time

Ambassador David MacNaughton describes what he feels was the toughest moment during his time as Canada's representative in Washington. 1:19

MacNaughton said the Canadian team was put in a tough spot because they were under pressure to sign a deal, since not being a part of the pact would have dire consequences for the Canadian economy.

The ambassador, who will leave his post at the end of the summer, said the Canadian government took the position that it needed to stand firm and fight for elements that had been left out of the Mexico-U.S. deal.

Chief among them was the dispute resolution mechanism that allows countries to seek outside adjudication if they felt another country was flouting the rules of the trade pact.

"There were a lot of sleepless nights," he said. "The thing that puts you under enormous pressure in these circumstances is you realize that you're dealing with real people's jobs, with communities, with the consequences of making a bad deal, [which] are very serious for a lot of communities right across the country."

In the end, MacNaughton said, the determination to stick with the strategy Canada had at the outset helped ensure that a deal beneficial to all three countries could be reached without leaving a sour taste in anyone's mouth.

"We were able to put it back together," he said. "We ended up getting the things that we needed to get out of the agreement."

"I don't blame the Mexicans for doing what they did. They did what they thought was going to be best for Mexico. And we did what we thought was best for Canada, and the Americans did what they thought was best for the United States. And what we were able to do was to actually find a win-win-win situation."

MacNaughton says Trump's 2018 announcement that the U.S. had struck a trade deal with Mexico violated an agreement that Trump's team would only negotiate bilateral issues with Mexico until Canada could return to the negotiating table. (Jason Burles/CBC)

MacNaughton also reflected on a recent story by Axios that revealed how Trump reacted to the cover of the May 1-7, 2017, issue of Bloomberg Businessweek that featured an image of Prime Minister Justin Trudeau headlined "The Anti-Trump."

According to the report, Trump tore off the cover and wrote across it in silver Sharpie a statement akin to "Looking good! Hope It's not true!" before sending it to Trudeau through diplomatic channels.

MacNaughton said he had to reach out to the White House to determine if the note actually did come from the president, a routine the ambassador said he had followed a number of times over statements attributed to Trump in the American media.