This would be pretty ridiculous and blatantly aggressive. And I'd love to see what the Wilson is doing internationally regarding the former Austro-Hungarian and Ottoman Empires. This would be nothing short of an American plot to try and become the world's sole superpower 25 years early. Except it's plain stupid, since the American citizens who supported WWI will be utterly confused why they're fighting with their former allies (Franco-American friendship and Italian-American loyalties notwithstanding), why they went to war to begin with, and American citizens who didn't support the war (and were suppressed by Wilson's anti-constitutional measures i.e. Eugene Debs and other Socialists, if not extrajudicially assaulted/lynched) will be even more enraged. Could the United States have secured a unipolar world in 1919 - early 1920s? Yes. But it would take some monstrous efforts on the home front since this is not a war the American people will support. You'd need some sort of American proto-fascism, which would mix various "ethnic" types (Italian, German, etc.) alongside the indigenous Anglo core of the US and promote American supremacy over the world, and this movement would need to be popular (at least as much as the OTL Socialist Party), to have any real base for this. Also, this would mess up the US financial system which had invested a ton of money in the British for their victory.
And what's the German response to this? They must be pretty pissed that the US
now decides to fight on their side.
It very unlikely, that this would play out realistically, but nonetheless we could have a few toughts about the events.
US would only invade Canada if Canada is joining in with the British.
I'm not sure that the British would drag Canada in to this war so soon. Canada is vulnerable, so defending it would be hard.
On the other hand the terrain and weather conditions would create a problem for the US troops. They would need a hell of an amount of soldiers to hold the lands. Constant revolts and partisan attacks would be a large problem for them.
Not really. Canada's major cities are very close to the US border, and in Western Canada have very easy terrain. The only problem is the Great Lakes which have been demilitarised since the early 19th century, but since the US controls the majority of the shipping on the Lakes they can more easily rig together some merchant cruisers before laying down actual lake battleships (probably monitors since the Canadians can't lay down anything to really oppose that). Whoever controls the Lakes controls Ontario and has easy access to Ottawa and the St. Lawrence.
As for the weather, don't do much in winter, plain and simple. Canada is not Russia. The Canadians have nowhere to retreat to, since their only major northern city is Edmonton (although Ottawa has some nice distance). Halifax and Cape Breton is also pretty defensible given the terrain (Isthmus of Chignecto) and Royal Navy support. There's a nice line of Vancouver, Calgary, Regina, Winnipeg, the Thunder Bay area, Windsor, Ontario, Ottawa, Montreal, Quebec, St. John, and Halifax which if occupied controls the vast majority of the Canadian population and economy with relatively few soldiers. The Canadian West is also sparsely populated and has many Americans as well as recent immigrants who can't really be counted on to join partisan bands. Although since the US is going blatantly aggressive and fascistic they might as well, and they'd be joined by Canadian socialists (supported by American socialists to some degree). On the other hand, we know partisan warfare does not work if no supplies are coming in. And the Royal Navy will have trouble shipping in supplies. But if we have ASB (since this is bordering on it) make the US population no worse than they were during World War I, this will be much easier.
The naval capabilities of Britain are more fearsome at this time if we consider the other world powers and their capabilities. A joint French-Italian-US armada could overpower them, but you could bet on it, that the British would not wait for the enemy to unify.
They would wreck the French fleet that is close to them and tie down the rest around Italy and South France. The only threat would be the US fleet, but it would be too much of a two-sided coin for them to engage the British alone.
The Pacific would be an overkill by the British and Japanese against US. They could unify their forces rapidly and the US would find its situation similar to the Eastern seas.
I think the optimal US strategy in the Pacific is to abandon the Philippines, Samoa, Guam, etc. and retreat to Hawaii, while harassing enemy shipping the whole way. Hawaii is almost impossible to take for the Anglo-Japanese. US shipbuilding accelerates and in 2 years the US has outbuilt both sides, and the counterattack across the Pacific begins.
In the Atlantic the US has a much easier time and should concentrate there to smash Canada and prevent any threat to the mainland (and Puerto Rico/Panama Canal). The US Marines take the Bahamas followed by an island hopping campaign aimed at Trinidad. Venezuela and Mexico cut off oil shipments to Britain and Japan under US pressure (the political situation--Mexican Revolution, Venezuela
caudillos--in both nations can easily be exploited by the US at this point). Along with the loss of US oil, the Royal Navy (and IJN) are now limited as to their available oil reserves. This is very, very bad for them. France and Italy have trouble in the Atlantic, but in the Mediterranean can concentrate on the British and deal with Cyprus, Malta, etc. within 12-16 months. They can likely recruit Spain and Greece as allies to assist dismantling the British in the area. Portugal stays neutral out of economic reasons, fear of fighting Spain, as well as losing the colonial empire and the Azores.
Japanese and British forces would take all French land in the Pacific along with the Philippines and take on the US islands one by one.
The US strategy would probably consist of sending either a large portion of the Eastern fleet to the Pacific or part of the Pacific fleet East to have an upperhand on one of the theatres.
Not without a fight. Philippines can be defended pretty well and will delay the Anglo-Japanese advance long enough to fortify Hawaii even more.
Utterly improbable that Britain and the USA ever go to war post 1814 if for nothing more than the damage done to each others trade such a war would bring
Something massive would have to have changed for this happen.
I disagree with that, but I think the US going to war with Britain in 1919 is beyond nonsensical. If the US really had that sort of ill-will toward Britain (or Canada, which will serve as the US punching bag for anger against Britain), then the US would have stayed neutral in World War I, if not taken active effort to prevent Britain from raising finances, importing food/supplies, etc. and helping the CP out just as much. Or at the very least helped negotiate a peace around 1917/1918 which would more or less be status quo antebellum yet pave the way for US influence in Europe and the colonial empires.
ASB. Nobody in their right mind wants to risk widespread revolt... For a few ships, especially.
The Senate rejects the motions of Wilson; Italy and France back down from this horrible, horrible idea. If the ASB make it pass, as soon as the orders are given to the respective Navies the Communists call for a successful general strike or three, and in Italy full scale revolution may well happen.
Much popcorn shall be passed around; the USSR has a field day & sensible minds will happily mind their own business.
Basically a realistic assumption. This is more a scenario which is fun to wargame but otherwise plain nonsense.