New Delhi, Aug. 25 -- Another trade war has started with a volley of tariff threats, and this time the fight is with America's neighbor and closest trading partner, Canada.

So far, there's no clear off-ramp to repair this critical relationship, and the back-and-forth threats will likely continue. Both companies and investors should buckle up for another bout of tariff-driven volatility-especially in sectors that depend most on trade between the two countries, such as autos, agriculture, and alcohol.

This weekend, both sides pushed for last-minute changes to a proposed trade agreement, ultimately upending a deal that would've staved off 50% tariffs on $20 billion, or a fraction of Canada's exports. However, the U.S. tariffs-now in effect...