I'm not a fan whizzing on our northern neighbor. They've always fought along side us and we hold positive relations as neighbors. That said, we've not held equitable trade though this should be managed via typical internal negotiations. However, that would discount the executive interest for the aggressive tariffs...
Loose Canadian/U.S. border.
One hell of a long border far, FAR longer and a fraction of border control than the short span along the Mexican border. Fentanyl among other trafficked values (inc. human) the southern border countries (MX and SA) supply our 350,000,000.
Here's the deal: Our various agencies intel is correct. Cartels exploit vulnerabilities. Access to Canada's population as well, and more importantly, the nexus to America's population. Seaports in Vancouver and Montreal are extremely weak for CBSA staffing. The land border between U.S. and Canada, 5,500 miles! The LONGEST bordering country in the world! CBP is up-ing their game, CBSA has begun to up theirs as well. Both are resilient though Canada is behind the times. Seaports in Vancouver to Montreal are over-run by the criminal element. Recently put to a stop (Thanks to Trump's rhetoric, as unfortunate as that sounds) Canada has begun to put a lid on immigrants to their country. Again, a great country and our valued neighbor. This is the nature of criminal organizations. Exploit weakness. Our Northern border is exposed due to the nexus through Canada.
As far as several countries that have taken advantage of America, it's about time. A few here throw around the words, "free trade"... this has not been the case and for far too long.
Reciprocal trade is fair trade. Not running companies out of the U.S. to maintain a competitive price with mass cheap, unregulated labor. We have become addicted to Chinese and other countries cheap labor. We should be paying more. That's the reality, IMO. There is an ethical value towards OSHA (if they don't get gutted), the safety and security our country offers our workforce. This cost money. China, on the other hand, cheap labor - and THAT is what we are addicted to.
In keeping as unpolitical as the OP requests, the command is a bully, much like a few bullies here. Personal ridicule attacks that certainly holds the opposite effect, though sure excites the follower's pom pom cheers, aside from strongarming countries to the table.