Gary Cohn, President Trump’s top economic adviser who reluctantly stood by Trump even after he failed to condemn neo-Nazis for the violence in Charlottesville, Virginia, last year, has finally had it and is resigning from his post. The thing that pushed him over the line? Steel tariffs.

Cohn has been fiercely battling the president and an influential contingent of anti-free trade advisers in the White House over Trump’s plan to impose sweeping tariffs on steel and aluminum import, warning that it could spark a major trade war.

He’d already threatened to resign last Thursday if Trump went ahead with the plan. But even though Trump ended up announcing the tariffs just a short while later, Cohn still stayed on in his position.

It seemed Cohn hadn’t given up quite yet, and in fact he was planning to make a desperate final attempt to change Trump’s mind by bringing leaders of industries that would be hurt by the tariffs to meet with the president at the White House on Thursday.

But Cohn apparently decided to abandon that last-ditch effort and go ahead and resign today. Stock market futures dropped after the news, a sign that the markets will experience volatility on Wednesday as investors see the odds of a trade war as increasingly likely.

“Gary has been my chief economic advisor and did a superb job in driving our agenda, helping to deliver historic tax cuts and reforms and unleashing the American economy once again,” Trump said in a statement. “He is a rare talent, and I thank him for his dedicated service to the American people.”

White House officials told the New York Times that there wasn’t one specific reason for Cohn’s exit. But the timing makes it clear that this is related to Cohn’s struggle to sway the president on trade policy.

Trump had explicitly snubbed Cohn recently. As Politico reports, Trump rejected Cohn’s arguments that tariffs would produce more job losses than gains during an Oval Office meeting earlier in the year.

Cohn’s exit will strengthen the anti-free trade bloc in the administration, which includes White House trade adviser Peter Navarro, Commerce Secretary Wilbur Ross, and US Trade Representative Robert Lighthizer.

With Cohn absent in debates over trade policy, it seems likely that Trump could become even brasher in his proposals to erect trade barriers around the American economy.