The administration is taking the action under a relatively obscure USA trade law meant to protect national security.
In retaliation to these tariffs, Canada's Foreign Affairs Minister Chrystia Freeland said Canada would impose surtaxes and other similar measures on steel imports, aluminum imports, and other products from the U.S. The surtaxes will cover $16.6 billion dollars, the exact amount of money made from Canadian exports of steel and aluminum to the U.S.in 2017.
The minister says she doesn't believe the tariffs will alter the government's plan to purchase new military gear from the USA because extra money is typically set aside for unexpected circumstances.
Minister Freeland is here to outline retaliatory measures.
On Wall Street, the Dow Jones Industrial Average is down 145 points or 0.62 per cent.
While China is seen at more risk from a trade war because its exports are larger than its imports from the United States, operations of American companies in China make substantial sales there, which could be hit by any turn in sentiment.
On March 23, U.S. President Donald Trump imposed a 25-per-cent tariff on steel imports and a 10-per-cent tariff on aluminum, but had granted temporary exemptions to Canada, Mexico, Brazil, the EU, Australia and Argentina.More news: Serena Williams Turns Things Around Against Ashleigh Barty
The European Commission's top trade official, Cecilia Malmstrom, said they had repeatedly pleaded with U.S. Commerce Secretary Wilbur Ross to forgo the tariffs and try to come up with a joint solution.
Canada and Mexico, embroiled in talks with the United States to modernise the North American Free Trade Agreement (NAFTA), responded swiftly.
"Most of the time as I travel around I get confirmation these are high quality high value steel imports", he said, adding that in many cases these were products that couldn't be obtained in the U.S.
Canadian Prime Minister Justin Trudeau said the Trump administration's argument that the tariffs were necessary for national security reasons was an "affront" to Canada, which has fought alongside the U.S.in numerous armed conflicts.
Mexico came out swinging after the announcement, immediately announcing that they will meet the metals tariffs with duties of their own on USA imports including pork bellies, fruit, cheeses and flat steel.
European Union executive Susan Danger said US companies in Britain oppose the decision and urged Trump to reconsider.
Reverse tariffs could be slapped on items like Harley-Davidson motorcycles.More news: Mexican journalist Hector Gonzalez Antonio beaten to death
But we see no sign of that in this action today by the USA administration. "We tried to do it through negotiation and we will do it by standing together and formulating a common European answer, possibly working more closely with Mexico and Canada", Peter Altmaier said. European Commission President Jean-Claude Juncker also said the EU would respond with penalties of its own on USA exports. Throughout these talks, the U.S. has sought to use the threat of trade restrictions as leverage to obtain concessions from the EU.
For the European Union, the measures will affect exports that were worth €6.4bn (£5.6bn) in 2017.
French President Emmanuel Macron called the move illegal. U.S. Chamber of Commerce President Tom Donohue warned in a letter to the body's board that current trade policies could threaten economic progress and cause the loss of more than 2 million jobs, mostly in states that voted for Trump.
Fox - a Brexiteer who has led calls for Britain to prioritise a trade deal with the United States after Britain has left the European Union - described the tariffs as a "patently absurd" move, and said the UK government was considering counter-measures. A handful of other USA trading partners, including South Korea, Australia, Argentina and Brazil, previously won permanent exemptions by agreeing to quotas on their exports.
Getting rid of the exemptions for Canada and Mexico, meanwhile, could complicate ongoing negotiations on NAFTA.
Tariffs of 25 percent on steel imports and 10 percent on aluminum will be imposed on the European Union.
Since Mr Trump's announcement in March, companies in the USA that buy metals have already reported higher steel prices and and complained that U.S. producers do not have the ability to meet demand.More news: 76ers agree to 3-year extension with Brett Brown | AP sports