The United States and Canada are barreling toward a deeper trade war after last-ditch efforts to strike a deal collapsed Friday.

Canadian Prime Minister Mark Carney vows to retaliate, says his country recognized that 'America has changed' and that the two countries would 'not return to our old relationship'

The United States is imposing 50% tariffs on $20 billion worth of Canadian products after last-ditch negotiations failed to resolve the latest strain in already tense relations…