Um... I never said anything about Canada or Mexico nuke the US... So yeah.... Ever hear of China?Right, because I'm sure Canada or Mexico would nuke the US.
The US has expanded from 13 colonies to 50 states (plus multiple territories) just fine...adding a few more wouldn't be a problem. Considering that Texas used to belong to Mexico, Florida to Spain, and many others... it wouldn't be entirely illogical for more to come.
And yeah, that expansion didn't happen over night... Not to mention, Canada alone is already larger than the whole US, so have fun?