Section 338 allows it. Anything Canada does that gives themselves any trade advantage allows retaliation. Essentially inevitable.

whitehouse.gov/fact-sheets/2026/07/fact-sheet-president-donald-j-trump-imposes-additional-tariffs-on-canada

The Trump Administration first used the International Emergency Economic Powers Act to declare a national economic emergency and impose tariffs on countries under the pretence that they were economic threats to America.

The Supreme Court ruled against the tariffs because while it allows the president to regulate foreign imports as a measure against economic threats; it does not specifically grant the president power to impose tariffs, duties or other taxes which is typically a power that belongs to Congress.

The tariffs on Canada are now being imposed under Section 338 of 1930's Tariff Act which allows the president the authority to apply up to 50% tariffs on countries the president finds has "discriminated", directly or indirectly, against the United States.

> I thought the supreme court said the tariffs are illegal without congressional approval

The ruling wasn't as generalised as that. What it said was that the president doesn't have the authority to impose tariffs under the provisions of the International Emergency Economic Powers Act, which Trump had cited as the basis for some tariffs. That made those tariffs effectively illegal, but the ruling said nothing about tariffs which had other legislative provisions as their legal basis. The tariffs that you see today cite some other rule as their basis rather than the IEEPA.

Even if it's illegal, who's stopping him?

Until anyone does, "crime is legal", as coffeezilla says.