British Republic
Banned
Over the course of the 'troubles' in NI the UK maintained an average of 9k troops there. About half of these could be seen as controlling the Unionists so that leaves 4.5k for the Republicans. The Catholic population was about 400k throughout this period (growing) and support for the armed struggle went up and down but was about 50% of the Catholic population and a tiny % of the Protestants.
So an organization with approx. 300 active 'troops' supported by 200k civilians needed 4.5k regular army personal on the ground to control it. Given the links between the two countries I think this is the most comparable situation.
Current population of Canada is 35mil. I think we can safely assume that if incorporated into the USA without the consent of the Canadian people supporting a resistance (not volunteering) would be near unanimous, so you would need? I make it 875k troops on active service in Canada at any one time.
OK the British Army were operating as riot police most of the time and all the fighting was down with the proverbial 'one hand behind their back'> Since then in Iraq and Afghanistan both British and American troops have used very different tactics. I still think given the land border and the fact that most Canadians could wander the USA passing as Americans this is the most comparable situation and it suggests things would be very nasty.
The IRA did almost all their killing with simple firearms and home made bombs so cutting off access to those in NA is out.
Well I was mainly talking about a ATL-WW1 Siutation but we can always bring it to OTL 2015...
Well firstly what needs to be factored in is why the United States has decided to invade Canada in the first place. Now if the US is still a democracy at this point then such a invasion would have to have at least a majority of the country supporting such a war and thus there would have to be a pressing reason why there is such support for such an invasion.
My view is that such an invasion would only have the needed approval if Canada ended up with such an Anti-American Government that it pretty much blocks all trade and gets very cozy with Moscow and Beijng (In other words like with Venezuela but worse). Now considering the closeness of US Cities to the Canadian Border that would give a National Security justification to push though an invasion (No one wants a Canadian Missile Crisis).
So lets say the invasion is done, the government is overthrown and the United States annexes Canada (Mainly because they don't want to go though this again, remember also that Canada is the biggest trading partner of America bar China). There is the question of what happens next.
I mean if the Americans co-opt the moderates who opposed the government and left them alone within the US Constitution then perhaps the worst that would happen is the odd terrorist attack here and there. If on the other hand they manage it in the sort of way they managed when it came to Iraq then you would need a lot of milltary force to deal with the blowback.
The question then comes to what happens from here, I feel it would get nasty but what do the Americans do about it. Especially when they have experenced what happens when you have a hostile Canada on your doorteep.