WASHINGTON (AP) — In firing up a trade war with Canada, President Donald Trump turned to a 96-year-old statute so obscure that many trade lawyers didn’t even know it was still on the books.

A week ago, Trump invoked Section 338 of the Tariff Act of 1930 to slap a 50% tax on $20 billion worth of Canadian imports. The move prompted dollar-for-dollar retaliation from Ottawa and strained already-tense relations between the neighbors and longtime allies.