Jamie Dimon, the CEO of J.P. Morgan Chase, made a grim warning about the impact of President Donald Trump’s tariffs, saying they threaten to drive up prices in an already slowing economy and do long-term damage to America’s alliances.

In his annual letter to shareholders, the head of Wall Street’s biggest bank said a recession is a serious possibility following the aggressive import taxes Trump slapped on China, the European Union and dozens of other countries whose products help keep U.S. consumer goods affordable.

“The recent tariffs will likely increase inflation and are causing many to consider a greater probability of a recession,” Dimon, one of the most influential figures in the financial services space, wrote. “And even with the recent decline in market values, prices remain relatively high. These significant and somewhat unprecedented forces cause us to remain very cautious.”

Trump may have some legitimate reasons to impose the tariffs, Dimon continued, but in “the short-term, we are likely to see inflationary outcomes, not only on imported goods but on domestic prices, as input costs rise and demand increases on domestic products.”

The tariffs present potential retaliation from other countries, wrote Dimon, who said his “most serious concern is how this will affect America’s long-term economic alliances.”

“America First is fine,” wrote Dimon, echoing one of Trump’s rallying cries, “as long as it doesn’t end up being America alone.”

These potential impacts on the economy would also drive up interest rates, he noted.

“All of these cross currents and turbulence may take years to play out,” he said. “It is almost impossible to confidently put them into a quarterly or even annual forecast.”

Dimon’s letter comes a day after billionaire hedge fund manager Bill Ackman issued a similar warning about the tariffs Trump announced last week, writing on social media that it’s akin to launching “economic nuclear war on every country in the world.” If Trump doesn’t pause the planned tariffs to allow for some negotiations over them, “we are heading for a self-induced, economic nuclear winter, and we should start hunkering down,” Ackman wrote.

“[W]e are in the process of destroying confidence in our country as a trading partner, as a place to do business, and as a market to invest capital,” Ackman added.

Despite stocks plunging, Trump has brushed off concerns. When a reporter asked him Sunday night how much he’d let the stock market endure, he replied: “I think your question is so stupid. I mean, I don’t want anything to go down, but sometimes you have to take medicine to fix something.”

Then, the president added, “I don’t think inflation is going to be a big deal.”