As others pointed out, it's not only the US economy that will be hit, but also everyone that trades with them.

To illustrate some examples, Canadian aluminum might end up with 25% tariffs. That means anything made within the USA that uses said aluminum will get a price increase. Canadian companies might end up with a surplus, since their main customers won't be buying as much (instead of paying 100 dollars for a tonne, 'mericans will pay 125 dollars per tonne). That surplus will drive prices down if they can't find someone else to buy the aluminum.

Since the tariffs aren't only on Canadian aluminum, but a lot of stuff from a lot of countries, some of that stuff will end up with a significant surplus and no new buyers. For smaller countries that rely on USA exports, that's going to hurt a lot.