Donald Trump has decided to ramp up a trade war with countries that would otherwise be considered American allies. The president has let exemptions expire on tariffs for steel and aluminum imports from Canada, Mexico, and the EU.

From tomorrow, duties of 25% on steel and 10% on aluminum will be imposed, which the White House insists are necessary for national security reasons. The reaction has been swift, with Mexico announcing retaliatory tariffs and the EU promising to follow through on its own measures flagged when the tariffs were first floated.

In the moments after the announcement, US-based steel companies saw their share prices jump. The five biggest listed producers have gained nearly $700 million in market value so far today, as their foreign rivals will face higher costs to ship steel to the US.

Steel stocks have done very well since Trump’s election. A key campaign promise was to protect US steel jobs and punish China for swamping global markets with too much steel. Since the election, an index of US steel producers has gained more than 80%, compared with a 26% rise in the S&P 500.

That said, it’s been a bumpy journey. The gains for US steel stocks today were big at the open, then quickly lost a bit of steam. Maybe this is because, in reality, there are few winners in a trade war. American workers in steel- and aluminum-consuming industries, like construction and auto manufacturing, are expected to shed jobs; steel workers that rely on speciality imported steel could lose their jobs; and the effect of retaliatory tariffs by trading partners will hit other American industries, especially agriculture.

How much steel can domestic producers sell if so many other industries are suffering?