Why did Donald Trump impose tariffs on Canadian steel and aluminum in 2018?

In May 2018, the administration of U.S. President Donald Trump announced the imposition of tariffs on steel and aluminum imports from Canada, Mexico, and the European Union.1

President Trump justified these measures by claiming that the tariffs were necessary to protect U.S. industries as a matter of national security. Canadian Prime Minister Justin Trudeau publicly rejected this rationale, characterizing the justification as "insulting and unacceptable".123

The move led to a period of heightened diplomatic tension between the two countries, as Canada responded with retaliatory counter-tariffs. The trade dispute became a significant point of contention during the 2018 Group of Seven (G7) summit, where disagreements over trade policy strained the relationship between the two leaders.413