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Tariffs are back—or at least the threat of them. Yesterday the Trump administration announced an additional 50 percent import duty on Canadian wine, hockey sticks, and other goods. But the tariffs won’t actually take effect for almost a month, which means they might never: Donald Trump has a history of retreating in exchange for minor concessions. So before you get too worked up, here’s what you need to know.

Wait, I thought the Supreme Court struck down Trump’s tariffs. How is he allowed to do this?

Good memory. The court threw out Trump’s import duties on many foreign products in February, saying he’d overstepped his authority to enact tariffs (which, with rare exceptions, are Congress’ turf). But the latest tariffs on Canada draw on a different presidential power, which permits “new or additional duties” on any country that “discriminates in fact against the commerce of the United States.” No previous president has used that provision to impose tariffs, though, so these may also face legal challenges.

OK, but why is Trump suddenly tariffing Canada again?

The administration says it’s payback for Canadian duties on American cars, dairy products, and alcohol. “Canada has been very, very tough on us over the years,” Trump told reporters today. But the backstory is important:........

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