When Canada was about to legalize mj a few years back, but the US said if you proceed we'll cut off trade. Canada would have gained lots of money from selling weed and the US would have lost more profits than Canada if trade were cut off.
I didn't know Canada was talking about legalizing a few years ago. But if true you're right. The U.S. could NEVER cut off trade with Canada. They know what a shit ton of money they would have lost and how many people they would have pissed off. The U.S. threatening to cut off relations with Canada is like going all in with a pair of 2's.