I would love to comment on the content of the article but paywalls are unfortunate.

So far US and Trump has been the only one using tariffs.

No. We have counter tariffs in place in Canada, and other countries have used counter tariffs in response to US Economic aggression as well.

Once we go this route other nations will employ the same tactics against us - what’s going to stop China from imposing tariffs on Canadian exports to China?

China already imposes numerous tariffs on Canada, and has routinely used them as weapons when we get into it with each other. This includes 100% tariffs on Canola oil, oil cakes, and peas; as well as 25% on pork and seafood products. Which were put in place in direct response to Canadian tariffs on China.

And why is Carney using tariffs at a time when there are serious threats to international trade regime?

The logic I have heard him give is this is meant to recoup some of the costs from tariffs against Canada, with the money gained from that going to support workers in the industries that are currently facing serious issues.

If he gave a different answer in the article, again, I cite the unfortunate pay wall.