Trump's tariffs on Canada, Mexico and China suggest he is willing to risk a global trade war. The main thing that could stop him would be a stock market selloff.

Business groups, economists and even some Republicans cautioned that new tariffs on goods from Canada, Mexico and China could undercut U.S. industry and the economy.

Trump knows the United States has more leverage over its neighbors than they do over his country. But exploiting that position of strength could cause a historic rupture.