President Donald Trump is invoking a 1930 trade law to impose whopping 50% tariffs on certain Canadian goods, threatening to reignite a trade war with the United States’ second-top trading partner.

The Trump administration imposed 50% tariffs on three categories of imports from Canada, dairy, alcohol, and cars, citing discriminatory actions.

The new tariffs will be imposed under an obscure legal provision, Section 338 of the Tariff Act of 1930, which allows the government to put tariffs on imports from countries that discriminate against ...