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 had expressed outrage at his plans, and experts 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.
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.
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.
What exemptions will there be from the new tariffs?
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 ahead of a cabinet meeting earlier on Thursday.
He also praised his country’s close relationship and trade surplus with Australia, saying “we’ll be doing something with them”.
The president also linked defence spending to trade, and said the US “subsidised” many countries” in the military.
He said there would be a reduction in tariffs for countries that “treat us fairly”, but said “many of the countries that treat us the worst on trade and on military are our allies”, singling out Germany for criticism.