His plan has sparked outrage around the globe, with investors spooked and stocks taking a dive.
Eight nations, including Canada, Mexico, Japan, Russia and Brazil, all sold one million tons, or more, of steel products to the US in 2016.
The world's top steel producer, China, is not among them. But it is one of the leading exporters of aluminium to the US, which now imports about 90 per cent of its domestic needs.
The EU says Trump's tariffs move amounts to protectionism and is vowing to take action to defend its interests. China is calling on the White House to exercise restraint in applying the charges. But Trump tweeted that trade wars are good and "easy to win."
So how close are we to all-out trade conflict?
Presenter: Elizabeth Puranam | Guests: Philippe Legrain - former special adviser to the Director General of the World Trade Organization; Einar Tangen - an investment banker who regularly advises the Chinese government on economic and development issues; Republican political strategist Joe Watkins, who served as a White House aide to President George H W Bush
