The moment Donald Trump slapped tariffs on Canada's steel imports was the moment Justin Trudeau realized he couldn't take the U.S. president at his word.

That hard lesson in the new reality of the Canada-U.S. relationship and several others learned by the prime minister are outlined in a new book by CBC senior writer Aaron Wherry.

The book Promise and Peril: Justin Trudeau in Power (HarperCollins) outlines the tumult of Trudeau's relationship with Trump, including a personal call to the prime minister to register a bitter complaint about Canada's Foreign Affairs Minister Chrystia Freeland, as well as Trudeau's initial stupefaction when the president launched a blistering Twitter attack against him from Air Force One hours after shaking his hand following last year's G7 summit in Quebec.

Like a lot of people, Trudeau was caught off guard by Trump's unlikely victory in 2016. Still, the prime minister decided early on to work hard on building a personal relationship with the new president by taking care to criticize Trump's comments, not Trump himself. "There was a discipline that I imposed on myself early," says Trudeau.

Working with extensive on-the-record conversations with the prime minister and his top advisers and with the perspectives of unnamed sources, Wherry describes how it quickly became clear to Trudeau just how little he was going to be able to control when it came to the Trump administration — and how badly Canada was going to need friends inside and outside the White House.

One of those diplomatic friendships, Wherry reports, was the product of Freeland's extensive contacts in the U.S., where she had worked for some time. Mutual friends in New York allowed the Trudeau administration to cultivate Jared Kushner, the president's son-in-law.

Unlike so many of Trump's advisers, Kushner would turn out to have staying power in the administration. He and Trudeau's chief of staff, Katie Telford, quickly built a rapport after a first meeting in New York. That relationship would turn out to be critical to getting a final NAFTA deal.

Katie Telford, Trudeau's chief of staff, left, was able to forge a strong relationship with Jared Kushner, Trump's son-in-law, that helped keep lines of communication open. (Sean Kilpatrick/The Canadian Press )

The deal came later. First, Trudeau had to make sure he survived a handshake.

The PM's first visit to Washington after the presidential election happened in February 2017. The meeting was carefully co-ordinated on both sides of the border to make sure nothing happened that would cause either government trouble.

Even that first public encounter between the two leaders, Wherry writes, was closely choreographed by Trudeau's people, who were fully aware that Trump's aggressive, dominance-signalling handshakes had become a point of interest for the media.

The physicality of Trump

"[Gerry Butts's] line to me was, 'Think of the physicality of it,'" Trudeau told Wherry. "The basic idea was to stay on balance. It was a topic of discussion on the flight to Washington. It's sort of silly that we would do that, but [the goal was to] make sure it's not a thing."

As it turned out, it wasn't a thing. But for Trudeau, that small win was short-lived.

The Trudeau government was about to embark on a very long campaign to renegotiate a trade deal with Canada's largest trading partner. Trump had made it clear early on that tariffs would be part of his approach to trade with all countries; he started musing about imposing them on steel and aluminum imports in the spring of 2017.

Trudeau attempted to use personal persuasion to protect Canada from the tariffs when he and Trump met at the G7 summit in Italy, Wherry reports. He raised it directly with the president, who then turned to Gary Cohn, his top economic adviser at the time, to insist that Canada be kept off the list of tariff targets.

Taking Trump 'with a grain of salt'

Perhaps Trudeau felt reassured by that exchange. But that feeling wouldn't last long. A year later, the Trump administration imposed the tariffs on Canada — and the prime minister had a moment of revelation about the nature of the man he was dealing with.

Trump "has moved into a place where whatever he says I need to take with a grain of salt, regardless of the handshake and the commitment and the look in the eye," Trudeau told Wherry.

It was the first of many unpleasant wake-up calls for Trudeau in his dealings with the mercurial Trump.

Another came during Trump's only trip to Canada, for the G7 meeting in Charlevoix, Que. Trudeau was focused on trying to hold the countries' leaders together and get them to a place where they could all sign a joint statement. Wherry's book describes the thorny behind-the-curtain negotiations on Iran and multilateralism that led to the final communiqué.

Trudeau and his team thought they had achieved success in getting Trump's agreement on a final communique at the 2018 G7 summit in Charlevoix, Que., but it was undone by Trump's Twitter attacks on Trudeau after he left the summit. (Christinne Muschi/Reuters)

As Trump and the other world leaders smiled for the cameras, Trudeau and his staff celebrated what looked like a win. Then, as the president was flying out to Singapore, he reportedly watched Trudeau at the closing press conference confirming Canada would impose retaliatory tariffs on the U.S.

An angry Trump tweeted that, "based on Justin's false statements at his news conference, and the fact that Canada is charging massive tariffs to our U.S. farmers, workers and companies, I have instructed our U.S. Reps not to endorse the communiqué …."

Trudeau told Wherry his first reaction was not to question the statement, or ask what Trump was talking about, but rather to wonder what he had done wrong.

"My first thought was, 'Oh my God, what did I say in the press conference? Where did I say more than I had before? What was it?'" he said. "'Cause I thought I did OK, I thought I walked the line."

Watch Justin Trudeau's closing press conference at the 2018 G7 summit:

Prime Minster Justin Trudeau speaks to reporters at the conclusion of the G7 leaders summit in Charlevoix, Que. 42:20

In fact, Trudeau hadn't said anything new. "Canadians, we're polite, we're reasonable," he told the closing G7 news conference. "But we also will not be pushed around." Still, Trump's attack dogs fanned out to attack Trudeau publicly. Peter Navarro, a White House trade adviser, said there was a "special place in hell" for the prime minister.

For Trudeau, it was one more piece of evidence that taking Trump at his word could be a tactical mistake.

Freeland a 'nasty woman'

Another unforeseen complication during the trade negotiations was Trump's open disdain for Freeland. "We're very unhappy with the negotiations and the negotiating style of Canada," Trump said publicly on Sept. 26, 2018. "We don't like their representative very much."

Foreign Affairs Minister Chrystia Freeland was the face of the government's NAFTA renegotiations and earned praise for reaching a deal. But she also drew the ire of Trump, according to the new book Promise and Peril. (Carolyn Kaster/The Associated Press)

Wherry reports Trump went much further in private. After the tariffs were imposed, Trudeau and Trump spoke on the phone; Wherry reports Trump called Freeland a "nasty woman" and accused her of talking about him to other world leaders.

The Liberals did work to make their positions very clear to the White House, using every tool they had. In September 2018, Wherry writes, Trudeau and his team decided they needed to draw a red line on Chapter 19, NAFTA's dispute resolution mechanism. The PM used a radio interview to state that Canada would insist on keeping the chapter intact.

It wasn't long before Prime Minister Justin Trudeau realized he would have to take U.S. President Donald Trump 'with a grain of salt,' according to a new book by CBC's Aaron Wherry. 4:29

The Trump administration was listening. "They appreciated how piquant our statement was on it, that the prime minister wouldn't focus the conversation on that issue unless there were strategic reasons for doing so," Butts, Trudeau's former principal secretary, told Wherry.

As negotiations intensified, the relationship between Kushner and Telford became even more important.

The book describes how, as the negotiating deadline came closer, Kushner suggested a "state of play" document would help determine where the two sides stood and how far they still had to go. Telford sent Washington the document; it was that analysis, and her working relationship with Kushner, that drove an intense few days of talks at the end of September that ultimately led to a deal.