I mean this is NOT a compliment when I say this, but Americans just don't know how fiercely nationalistic Canadians are. They will reject that descriptor out of hand, but they are absolutely united in the idea that Canadian is a cultural identity, that is is distinct from American, British, or French identities, and that their culture is more or less the secret 5 star restaurant you don't tell friends about so they don't shit up the place with their basicness. They will fight us. To the death.
Whatever you may believe about the rest of the world "wanting to be Americans" it is adamantly not true of Canadians. I have been told NUMEROUS times that "we don't want to be you, you know" totally unprompted. It's a national ethos to not be us. They will fight us, with everything they have.
Now, will they fight an invasion? That's a whole different kettle of fish. I am VERY concerned about what happens if we send in the tanks because I simply don't believe Canada has the capacity to draw us into an Afghanistan style occupation situation. But given that right now we're talking about using American soft power to simply take Canada - let me say, they will NEVER accept that. You will have to send troops. It's a nonstarter. They don't WANT to be us. They want to be "like us but better."