Canada

Tariffs are used by countries very commonly and isn’t a Turnip original (yeah I spell his name however I’m feeling like).

The reason why tariffs are used is generally to protect a local industry, typically one that’s weak, either because they’ve fallen behind, or that it’s a new budding industry and fierce competition will just obliterate its chances of even growing.

Think of, say, our EV sector, which is essentially in its infancy. Sectors like these tend to have higher prices on their products because they’ve not reached a critical point and can enjoy the benefits of economies at scale, where they’ve can operate efficiently while mass producing their products; they’re still trying to figure out what works and works well, and so their stocks are limited, which usually means that their means of production is also not as efficient as an established company or sector. Now imagine if we don’t have tariffs against Chinese EVs, which is a mature Chinese industry at this point, and still enjoys massive government benefits to subsidize a lot of its production processes, i.e. their costs are low and they can sell their EVs for cheap. Imagine them just exporting a bunch of EVs to Canada. Without tariffs inflating their prices, most people would probably just buy Chinese EVs cause, well, that’s the economic thing to do individually in these unstable times. Our local EV companies would easily be beaten by cheaper Chinese rivals, especially when our own quality is not even close to competing with Chinese standards, i.e. they can’t make enough of revenue to cover their business operations and ongoing developments, and so the local industry would either die off because people may be discouraged to work in that sector, or it’ll take forever to grow.

If you look around what other sane countries are doing, you’ll see that a lot of them have some level of tariffs in various industries levied against enemies and allies alike, because, well, you, as the government, generally have an incentive to protect your local industries, or people would have a hard time finding jobs and earning money, and if they can’t do that for a long enough, you can kiss your government seat goodbye.

To what extent is there non-tRump precedent for this

If you're asking about steel spcifically I don't know, but our countries have had tariffs against each other for a long time (off the top of my head, Canada on Chinese EVs, China on canola)

I wouldn't say this is adopted from the Florida man's playbook, although it may look so at first sight.

As Mr. Carney says, two-thirds of the steel consumed in Canada comes from abroad (with a large part from China). In the U.S. this rate is one-third, in the EU one sixth. This makes Canada dependent on foreign suppliers.

International trade is fine if everyone plays by the rule of law. If not -which is arguably the case in the predictable future- this dependency puts your economy at a higher risk, and it may make you vulnerable to coercive political measures which we have been observing exactly by countries like China. So protecting your own industry and collaborate with those countries with whom you have a free trade agreement could be a good move.

But that's just my interpretation. Maybe I am wrong.