Donald Trump stands accused of firing the starting pistol on an international trade war after pressing ahead with his threat to slap tariffs on EU steel imports.
Here we look at what it all means.
The US steel industry, Mr Trump hopes.
Tariffs on imports should trigger increased demand for US steel, which will be welcomed in the Midwest 'rust belt', where Mr Trump got a lot of support at the 2016 election.
However, US steel consumers would have less choice and could be forced to pay more for steel products, which could mean higher prices.
EU exports potentially subject to US duties are worth €6.4bn. So the industry fears the tariffs could cause serious damage.
Unless Mr Trump backs down, a trade war seems inevitable.
The EU has already prepared a list of US imports for retaliatory tariffs, including bourbon, blue jeans and Harley-Davidson motorbikes.
And like military wars, trade wars have a tendency to escalate.
Tariffs on imports generally mean higher prices for consumers.
And according to most economists, trade wars are not usually good news for overall global prosperity.