When asked this week what he had learned about Donald Trump during the US president’s first months in power, Justin Trudeau paused.

“I’ve learned that he listens,” the Canadian prime minister replied after a few seconds. “He is a little bit unlike many politicians.”

As laughter broke out among the audience at the public Q&A with Bloomberg News, Trudeau continued. Politicians are usually trained to remain on message, he said. “[But] he has shown that, if he says one thing and then actually hears good counter-arguments or good reasons why he should shift his position, he will take a different position, if it’s a better one, if the arguments win him over.”

Trudeau’s comments may help to explain how this week, Canada unexpectedly joined the list of countries in the president’s crosshairs. On two separate occasions Trump took aim at Canada’s protectionist trade policies, describing them as a “disgrace” and “unfair”.

It was a sharp reversal from a few months earlier, when Trump assured Canada that his administration would simply be “tweaking” the North American Free Trade Agreement (Nafta). “We’ll be doing certain things that are going to benefit both of our countries,” Trump said at the time. “Our relationship with Canada is outstanding.”

His words unleashed a sigh of relief in Ottawa, where officials had spent months laying the groundwork for a smooth relationship with the new administration. Few countries have as much at stake when it comes to US relations; Canada sends about three-quarters of its annual exports to the US, and nearly 400,000 people a day cross the shared border. Roughly 2.5m jobs in Canada depend on trade with the US.

The relief, however, was short-lived. On Tuesday, as he unveiled his “Buy American and Hire American” executive order in Wisconsin, Trump called out Canada and its reliance on import tariffs and production controls to protect its dairy sector. In Canada, Trump said, “some very unfair things have happened to our dairy farmers and others.”

He did not elaborate, but his comments were believed to reference a collective price drop by Canadian dairy farmers to compete against imports of ultrafiltered milk, a high-protein concentrate that had been flowing across the border from dairy producers in Wisconsin and New York.

Trudeau and Trump – frenemies? Photograph: Reuters

“We are also going to stand up for our dairy farmers in Wisconsin,” Trump said. “And that demands, really, immediately, fair trade, with all of our trading partners. And that includes Canada.”

Later in the week Trump reiterated his stance, this time widening it into a pointed attack on several sectors north of the border. “Canada, what they’ve done to our dairy farmworkers is a disgrace,” Trump said at the Oval Office.

“We can’t let Canada or anybody else take advantage and do what they did to our workers and to our farmers … included in there is lumber, timber and energy. So we’re going to have to get to the negotiating table with Canada very, very quickly.”

Trump also reiterated his attack on Nafta, this time adding Canada into the mix. “The fact is, Nafta – whether it’s Mexico or Canada – is a disaster for our country. It’s a disaster. It’s a trading disaster,” he said.

Canada’s top envoy to Washington was swift to respond.

“Canada does not accept the contention that Canada’s dairy policies are the cause of financial loss for dairy farmers in the United States,” David MacNaughton, Canada’s ambassador to the US, wrote in a letter on Tuesday evening. Instead, he linked any issues in the US dairy sector to US and global overproduction.

The letter was to the governors of Wisconsin and New York, who had come together recently to pen a letter to Trump asking him to address Canadian dairy practices.

Canada’s finance minister, Bill Morneau, said on Friday that the change in tone – which saw the president’s view of Canada-US relations go from outstanding to a disaster in the span of two months – was to be expected.

“It’s really not a surprise that we’ve moved from a positive relationship, which it is, to thinking about specifics,” he told reporters during a visit to Washington, according to the Toronto Star.

Trudeau – speaking on Thursday before Trump launched into his second swipe against Canada – played down the president’s comments. “Now I understand how certain governors are speaking to certain constituencies on that. It’s politics,” he said. “The US has a $400m dairy surplus with Canada, so it’s not Canada that is the challenge here.”

He continued: “We’re not going to overreact, we’re going to lay out the facts and we are going to have substantive conversations about how to improve the benefits for citizens on both sides of our borders.”