Stock investors largely stopped fearing tariff headlines after the S&P 500 shrugged off Liberation Day and climbed to record highs, but the latest U.S. and Canada standoff carries a twist that could make this round of trade war pain very different from the last.

Canada's Prime Minister Mark Carney said on Saturday ​that starting September 8 Canada would impose tariffs on imports from ‌the United States across a raft of sectors in retaliation for President Donald Trump's new 50% tariffs that came into effect from midnight.