If Canada gets any sort of prep time it can gut the US. Look at the http://en.wikipedia.org/wiki/Northeast_blackout_of_2003. That was accidental. Given how integrated the power grids are in North America Canadian operators deliberately trying to crash the grid could cause all sorts of shenanigans. Hell all we have to do is wreck the machinery at Niagara Falls and we could concievably black out New York State for months. Similar shenanigans could be done with both fresh water and oil supplies. The potential economic damage is catastrophic. Actual military resistance would be minimal, I'm sure the US could effectively destroy our military in the space of 24 hours. Actual armed partisan style warfare would be pretty minimal too, outside of places like Northern Ontario and whatnot the neccessary population with the neccessary skillset is nonexistent. As far as the type of resistance that the US is hopelessly incompetent at dealing with, our population could fuck with them endlessly. Aside from the passive resistance shenanigans where we do as little as possible for our overlords and throw as many monkey wrenches as possible into the works, we have a very well educated urban population. Expect a never ending letter, car, and anthrax bomb harrasment campaign. Canucks can blend almost perfectly into a typical American population to boot, which is an absolute security nightmare for the latter. America is demonstrating an absolutely Epic Secure Border Fail in keeping Mexicans from swarming over the southern border, there is no way they can prevent the occasional Canadian from slipping across the northern one to do bad things on US soil. Given America's proven arrogance and Bull in a China Shop approach to COIN operations, expect them to make the same classic stupid moves over and over and over as they gun down unarmed Occupy Style anti-American protests, round up and execute hostages, etc, etc, in such a way that they continually replenish and remotivate the supply of Canadians willing to blow up shit. In a nutshell with typical American behaviour Canada becomes America's Northern Ireland. Longterm this is a net loss and continual drain for the Americans. A decade later expect a humiliating American withdrawl. Now if the Americans got with a VERY unAmerican soft occupation style, more of a merger or Anschluss, the results are much better. The vast majority of Canadians are politically pretty far to the left of the US Democrats; Obama would be considered a hard Right Conservative in Canada. This means that either the US political landscape suddenly gains a viable third socialist party, or it means that the Democrats now have a majority in Senate and possibly house, forever. Canadian politicians are used to working in a much more sophisticated political landscape than the Baby's First Parliament two party situation in the US. If you get a viable third party consisting of a majority of Canadian voters and all the currently disenfranchised left wing voters in the States (people like the OWS) neither Democrat or Republican can gain any sort of majority without their support, this allows an endless game of Kingmaker for the far left party. If the former Canadians get Big Tented into the Democrats, they will have an effect on them similar to the effect the Tea Party crazies have on the Republicans, only it'll force the Democrats to make a serious hard left turn to placate them. Either way it forces the political landscape to shift leftwards. The rational parts of America slowly get more and more Canadianized. What happens after that, I don't know.