The US could simply dust off one of its old War Plans, and modify it to just aim at Canada.
There's also the fact that the US could also swoop into the Yukon, NWT, and Nunavut from their position in Alaska.
With that aside though:
Why Canada bought those is kinda a headscratcher. If Canadians have to use those:
- the US and Canada are at war
- someone attacked the US, Canada, or Mexico and we're all pissed
- "Seriously, you actually did that?" occured and nukes are flying
The German Leopard tank was developed during the Cold War, and given that Canada and what was then West Germany were both NATO members, the US didn't really raise a fuss about it. Now on the other hand, if Canada were to field a Soviet built tank, that would be a totally different issue. Considering the level of Canada's economy, their military expenditure is on the same level as the Baltic States.
Perhaps the most ironic thing about the US military industry is that while they're basically the biggest cash cow, it's that their equipment is far too expensive to produce and maintain. If they would simply just streamline some of their military technologies and go for the whole "easy to produce, easy to maintain" mentality, along with simplification of various weapons, not only could they still earn profits from the sales of the streamlined and simplified US military weapons and equipment, but that they could lower the price of them, making the demand for the streamlined and simplified US military weapons go through the roof. Basic economics.
For example, the Stingray light tank that is used by Thailand, Canada could have told the US that they're interested in purchasing that tank, instead of going for the more expensive Leopard. Being a naive fool that I was back then, I asked a Canadian soldier why they don't just purchase T-72 battle tanks, before the soldier told me that they're mass produced junk.
And that's not getting to the issue of the Canadian Air Force and Navy as well, with the issue surrounding HMCS Chicoutimi and the attempt by Doughboy to replace the CF-18s. Once again, my emphasis on streamlining and simplifying military equipment could have been applied to the failed F-22 fighter plane and would have been earmarked for foreign markets, instead of the notoriously delayed F-35.