New Delhi, Aug. 22 -- When Canadian officials sat down with their American counterparts on Thursday, Trade Minister Dominic LeBlanc emerged from the talks describing them as productive. Less than 24 hours later, negotiations had collapsed, and the United States had imposed 50 percent tariffs on $20 billion worth of Canadian goods - using a provision of American trade law that has never been invoked to levy tariffs in nearly a century of its existence.

the country's largest trading partner and a fellow signatory to the United States-Mexico-Canada Agreement - marks a rupture in North American economic relations with no modern precedent.

The tariffs, which took effect at midnight, cover a politically curated list of Canadian exports: hocke...