"Tariffs are actually, we've had a lot of experience with them. They're an act of war, to some degree," Buffett said.

President Donald Trump’s long-threatened tariffs against Canada and Mexico finally went into effect Tuesday, putting global markets on edge and setting up costly retaliations by the United States’ North American allies.

Trump's Commerce secretary said in a CNN interview that Canada and Mexico still need to do more to stop fentanyl flows into the U.S.