“By placing massive and disproportionate tariffs on our friends and our enemies alike and thereby launching a global economic war against the whole world at once, we 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 wrote.

He argued that Trump should consider calling a “90-day time out” that would allow him to negotiate and solve “asymmetric tariff deals, and induce trillions of dollars of new investment in our country.”

If that doesn’t happen and instead the U.S. launches “economic nuclear war on every country in the world, business investment will grind to a halt, consumers will close their wallets and pocket books, and we will severely damage our reputation with the rest of the world that will take years and potentially decades to rehabilitate,” Ackman wrote.

“What CEO and what board of directors will be comfortable making large, long-term, economic commitments in our country in the middle of an economic nuclear war? I don’t know of one who will do so.”

Ackman predicted that if markets crash, “new investment stops, consumers stop spending money, and businesses have no choice but to curtail investment and fire workers.”