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.

Tariff retaliation: Mexico and Canada immediately vowed to impose tariffs of their own. The Canadian prime minister, Justin Trudeau, announced retaliatory tariffs starting with 25 percent tariffs on approximately $20 billion worth of U.S. goods on Tuesday, with $85 billion more to follow within three weeks.