Donald Trump hit back at Justin Trudeau's tariffs with a fiery claim that Canada will "cease to exist as a viable country" without U.S. subsidy. The U.S. president ignited a bitter trade war, with Mexico and Canada retaliating after he imposed hefty levies on both nations.

President Donald Trump said the U.S. pays hundreds of billions of dollars to "subsidize" Canada, and Canada can avoid tariffs by becoming the 51st state.

Experts say the 1977 International Emergency Economic Powers Act is untested for imposing import tariffs and Trump's action will likely face swift court challenges that could set important precedents.