Donald Trump is slapping tariffs on EU steel and aluminium imports - kicking the bloc where it hurts.
The move is sparking warnings that it could destroy Europe's steel industry, where prices could drop by up to 40 percent.
The European Commission's President, Jean-Claude Juncker, has described the tariffs as "protectionism, pure and simple."
While the EU Trade Commissioner added: "This is not the way we do business."
In a tweet, EU parliament group leader Manfred Weber said: "We will not accept this highly regrettable decision without reacting."
And that means there'll now be extra duties on American imports into the EU.
It spells uncertainty for businesses on both sides of the Atlantic - including Europe's steel sector.
"We need to take safeguard measures for the EU. We're at the front door of a trade war," said Axel Eggert, Director-General of the European Steel Association.
"Therefore we have to continue discussions, negotiations with the US. That has to be done on a political level because the European Commission is also planning other re-balancing measures, it's not only about safeguards.
The steel industry is big business in the EU.
It directly employs 320,000 people, producing 170 million tonnes of steel every year, at more than 500 production sites.
The US is the second biggest export destination after Turkey, 4.9 million tonnes of steel went there last year.
US imports like jeans and Harley Davidsons have been earmarked for the EU's retaliation measures.
Brussels will also now trigger a dispute settlement case at the World Trade Organisation, saying the US measures go against agreed international rules.
All this, amid a worldwide glut of steel and aluminium largely blamed on excess production in China.
"There is a much broader issue, a much bigger issue that the administration in Washington is aware and that's the distortions in the Chinese economy that need to be addressed. If those issues are going to be addressed, the US will need to have the EU as an ally. And to be getting into a quarrel with the EU at the same time that we have a much bigger issue is tactically wrong - and strategically wrong," commented Peter Chase from The German Marshall of the United States.
The EU has said it doesn't want a trade war, but it's difficult to see that now being averted.