[impimp]

Why would Trump get protectionist on our ally for a mineral we need here in the US?

[conservative98]

Because Canada is not our ally, merely neighbors and a socialist country who hates us and take from us.

[roadcat]

Why would Trump get protectionist on our ally for a mineral we need here in the US?

I can see a reason because of how Canada treats us:

At that time, Canada retaliated with C$16bn in tariffs on American products, targeting items manufactured in key Republican-held electoral districts including ketchup, bourbon and lawnmowers.

[Rockingham]

Over the last generation, domestic US aluminum production has collapsed due to imports from China, Canada, and other countries, with illegal Chinese state subsidies driving the process worldwide. For the sake of national security and our aerospace industrial base, aluminum alloys of high quality and advanced composition are critical strategic materials that the US ought to be able to develop and produce here instead of having to import them from Canada or elsewhere.

After all, no matter how reliable Canada is as an ally in general, every now and then, a Leftist government comes to power in Canada and insists that on one or more particular issues, the US must yield to Canadian interests -- or else. The fewer such points of vulnerability for the US, the better off we are even when Canada is on the other side of the table.

[enumerated]

Because Chinese aluminum is being routed through Canada to evade import tariffs.