President Trump has signed controversial orders imposing heavy tariffs on steel and aluminium - but some countries will be spared.
Mr Trump has said the US is suffering from "unfair trade" and that the move would boost US industry.
But countries have expressed outrage at his plans, and experts have warned of new trade wars.
The tariffs will go into effect in 15 days and include exemptions for Canada and Mexico.
Tariffs of 25% are to be placed on steel and 10% on aluminium imported into the US.
The tariffs are opposed by many in the president's own party and by the US's major trading partners.
Republican Senator Jeff Flake - a prominent critic of Mr Trump who opposes the move - said he was drafting legislation to nullify the tariffs, saying trade wars are only ever lost.
House Speaker Paul Ryan said he disagreed with the action and feared its unintended consequences.
What has the international reaction been?
EU Trade Commissioner Cecilia Malmstroem tweeted that, as a close ally of the US, the bloc should be excluded from the tariffs
French Finance Minister Bruno Le Maire said there were "only losers" in a trade war and that France regretted the US announcement
The British government said it would work with EU partners to consider "the scope for exemptions" while "robustly" supporting UK industries
Steel and aluminium workers were at the signing.
Mr Trump praised them as "the backbone of America" - and alluded to their role in his election.
He said such workers had been betrayed - but that was now over and he was delivering on a campaign promise.
The president said the tariffs would defend America's national security.
They will apply to all countries except Canada and Mexico, which will be exempt while discussions over the North America Free Trade Agreement (Nafta) take place.
There are provisions within the documents for other countries to get exemptions.
"We are going to be very fair, we're going to be very flexible," Mr Trump said earlier on Thursday.
Mr Trump says he aims to protect the American worker - and he promised in his campaign that he would rebuild the steel and aluminium industries.
The two metals are "the bedrock of our defence-industrial base", the president said on Thursday.
The industry has been "ravaged" by aggressive foreign trade practices that are "an assault on our country", the president said.
He is also making a political decision, aiming to appeal to blue-collar voters in states such as Pennsylvania - voters who turned away from the Democrats to support Mr Trump in 2016.