This troubles me. The USA is the last group you want running a One World Government. Just how long do you think we have until belonging to this One World Government is mandatory and enforced by the US? Oh, and Canada will not become the USA's brain. No US citizen would allow it. Do you know how much Canadians and Americans despise eachother? Hell, Canada might even have a bloody civil war over this.
I said, "want", not "expect to happen". Here's the checklist for a merger to happen.
One, it has to be peaceful. I don't think either side would even consider force, but hell, the American military is pretty notorious for bad ideas. Violent takeover=lots of protests, the UN bitching, and major problems with the rest of the world.
Two, the leaders who try this have to be really well liked by both sides. Since American presidents aren't generally liked by the rest of the world (or by Americans, nyuck nyuck nyuck), that's the first major hurdle. Americans don't generally like any foreign leader from any nation, so good luck with that.
Three, both sides would have to shift a bit more in political leaning until they're closer to each other. Basically, the US needs to be more liberal and/or Canada needs to be more conservative. Ideally their leaders would also coincide on polical stances. This probably means a conservative for Canada and liberal for the US. Us Americans sure are conservative.
Four, the two countries would need to have more in common. One currency, one language, ect. There's already a lot to go off here, but one currency is a must. Beyond that the US needs to go metric and provide social services in a similar manner to Canada (or Canada needs to cut social services, but I somehow doubt that'll ever happen).
Five, the idea needs to be floated and given a lot of time to digest. Probably at least a generation for both sides to accept. Canada wouldn't be too bad, but Alaska and the US South would bitch and moan til the end of time, but I think they'll be won over with the right campaign. Places close to Canada will probably be cool with the whole deal.
Six, there would have to be some force driving the two countries together. No, special interests groups and big business are not enough. Terrorism is good but limited to military and transportation cooperation. Revolt might work, but I don't see that happening in the near future in either country. If China or Russia was more militaristic I could see the rivalry being the catalyst. Alien attack would certainly suffice, but those damned aliens never arrive on schedule.
Anything I'm missing? Probably, I'm no expert.