We are one people (other than Quebecois) divided by an arbitrary border. In some sort of nationalist upheaval I could Americans wanting to "reunite" with Canada or something like that.
Or if the midwest grain fields stop yielding enough during a global drought, the USA might turns its eyes to newly thawing land up north.
It would look more like Germany taking over Austria than Germany invading Poland in any case.
I don't see the US taking over Canada before a World War would break out, however, which I think is a bit more of a realistic mitigating factor.