The US, and that's really about it.
Anyone else tries it and Mexico basically has the entirety of North and South America saying "we've got your back".
Mexico's plans are pretty similar.
Basically: "Invade us and the big bully to the North who doesn't particularly like Hispanics will be extremely upset with you."
Cold War? Try even older.
The US and Canada actually do have pre-WWII plans for fighting each other that are available to the public.
Canada's: Defense Scheme #1
America's: War Plan Red
Both were kinda sorta "oh shit, we actually have to do this" contingency plans.
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
If history is any indication the Canadian Military is a much meaner and nastier version of the US military because you get the US volunteers who want to fight while the US is still sitting on its ass and deciding "will we, or won't we".
As long as Canadian military equipment is similar to US...