Billionaire hedge fund manager Bill Ackman has warned that World War III has “likely started already” amid Russia’s invasion of Ukraine.

Ackman, the CEO of Persing Square Capital, said that “WWIII has likely started already, but we have been slow to recognize it,” although he added that there was “much more we can do before we enter a hot war with Russia.”

The U.S. “could stop the absurdity of buying oil from Russia and funding the war, [and] Europe could follow suit once demand for gas declines in the Spring,” Ackman said.