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New American tariffs come into force in earnest today on $20bn-worth of Canadian goods ranging from ice hockey sticks to false moustaches.

So what? This is war. So said Mark Carney, Canada’s prime minister, in a speech on Saturday that may be remembered as his most important since January, when he told the world’s middle power countries to stop sucking up to Trump and stand up to him instead.

That was about Greenland, and $20bn isn’t much compared with total US-Canadian trade (worth $900bn a year) or with the value of Canadian exports of steel, aluminium, cars and timber on which Washington has already imposed 25-50% tariffs. Even so, these new levies are a critical test of

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Carney’s nerve as a champion of the threadbare international rules-based order;

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Trump’s use of the 1930 Tariff Act to keep bullying US trading partners, after being told by the Supreme Court he can’t justify tariffs as a response to a national emergency; and

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Trump’s appetite for tariffs generally as Trumponomics falters, US debt soars and inflation threatens to punish Republicans at the midterms.

Carney’s beef. Where to start? “America is trying to break us so they can own us,” he says of Trump’s continued taunting about Canada as a 51st state. He ordered his negotiators to pull out of trade talks late on Friday, accusing the US of throwing in last-minute deal-breakers on cars, trade policy and culture. Trump’s point-person said Canada was to blame.

Why the talks? Carney proposed them with a view to a general reset after 18 months of trade skirmishing and Trump’s announcement of the new tariffs last month. (Then as now, Trump may have been looking for distractions from his Iran fiasco – in the interim he has also picked on Oman and South Korea.) Carney doesn’t try to hide his irritation with White House efforts to sabotage a trading relationship that earns the US $1.6bn a day.

The detail. On cars, Carney said the US wanted to keep tariffs on big pick-ups and “light trucks” made in Canada. On trade policy, it wanted to limit Canada’s ability to make its own deals with other countries. On culture, it wanted to limit Canada’s official use of French.

Between the lines. By Thursday, both sides thought they were close to a deal. US negotiators led by Jamieson Greer, a Trump loyalist, were optimistic. Theories on what happened next include…

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a spanner in the works from Howard Lutnick, the US commerce secretary, thought to have made late interventions on behalf of US auto makers and aluminium smelters;

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a “power play” (Carney’s words), picking on Canada’s trade deals and use of French because those are things Washington would meddle with if Canada were a US state; and

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Canadian politics: Francophone separatists lead in polls before a provincial election in Quebec, and Carney has to be seen to be foursquare behind the Québécois.

To note. Carney hasn’t always been uncompromising towards Trump. He has caved on taxing American tech firms and streamers and most notably on the $300m Gordie Howe Bridge across the Detroit River – paid for entirely by Canada but opened last month only after Ottawa let Washington take half the tolls.

The upshot. “Dollar for dollar” retaliatory Canadian tariffs come into force on 8 September – unless Trump blinks. It wouldn’t be the first time if he did, but if he doesn’t then a full-blown trade war could ensue in which David Frum (who is Canadian) suggests the question won’t be how much pain each country can inflict, but how much it can endure. In that scenario it could make a difference that nearly two thirds of Canadians back strong opposition to US tariffs; and that Carney has a 60 per cent approval rating to Trump’s 33.

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What’s more… Carney has two nuclear options: denying the US access to Canadian critical minerals – vital for weapons, satellites and turbines – and energy. No wonder he radiates confidence.

Photograph by Dave Chan / AFP via Getty Images