President Trump has invoked Section 338 of the Tariff Act of 1930, a long-dormant provision that allows the president to impose duties in response to unreasonable and discriminatory trade practices, to levy a 50 percent tax—the maximum permitted—on a small subset of Canadian imports. The move begins a new trade war with Canada and could expose the administration to another court challenge. No president appears to have used Section 338 before Saturday, while later laws adopted by Congress have more clearly defined presidential tariff powers, prompting trade lawyers to question whether the provision has effectively become obsolete. The legal uncertainty follows repeated defeats for Trump over tariff authorities, including the Supreme Court’s February decision striking down his signature so-called reciprocal tariffs.