U.S. refineries rely on 4 million barrels of oil a day from Canada, and American farmers depend heavily on Canadian potash fertilizer.

Canada struck back at U.S. President Donald Trump on Tuesday with tariffs on about $20 billion worth of U.S. goods. The outcome could resonate far beyond Canada.

Canadian Prime Minister Mark Carney has shown he will not be pushed around by his neighbour. He is taking a big risk.