U.S. President Donald Trump‘s latest tariff threat against Canada is based on a nearly 100-year-old trade statute that has never been used to impose tariffs before — and not even goods covered under the Canada-U.S. Mexico Agreement on free trade (CUSMA) are safe.

Trump signed three executive orders Monday to impose a 50 per cent tariff on many Canadian goods under Section 338 of the U.S. Tariff Act of 1930, which allows a president to tariff any country that “discriminates” against U.S. commerce.

In this case, Trump took aim at Canada’s retaliatory tariffs on U.S. autos and the provincial and territorial boycotts of American alcohol, as well as Canada’s longstanding supply management system for its dairy industry that limits foreign imports.