My opinion is that Canada's plan to attack and damage nearby American cities, (damaging some limited industry) I also think that Canada would be overrun but after WW1 they would value there independence and firmly 'Canadian' identity, so I can see it being a very hard occupation for the Americans.
This leads to the Royal Navy vs the USN - Assuming outbreak of war between 36-39
Carriers - Britain and America both have a similar number of Carriers at the outbreak of the war, with Britain having marginally more in construction. American Carriers have superior deckspace, and thus plane numbers but are optimised for Pacific rather than Atlantic operations. American carrier based planes are superior to British carrier based planes. British Carriers having slightly less planes, but -much- thicker armour and armoured decks while also being optimised for Atlantic/North Sea operations would have a slight advantage, but would be let down by inferior carrier based planes. Historically British Carrier aircraft were not much of a focus as efforts were to build and maintain superior land based aircraft to win the Battle of Britain - If this were to become a Battle of the Atlantic I think we would see after the early stages of the war superior/equal to American planes due to this being the focus of British Scientists.
Other Capital Ships - Royal Navy has more, but more outdated capital ships than America, leading to America having a theoretical edge, HOWEVER despite being outdated British ships did have superior Anti-Aircraft Guns, and naval guns nullifying that edge.
Other Ships - Much like capital ships that pattern of older but better technology seems to be apparent throughout other ship classes.
This leads us rather neatly onto modern technology. To be frank Britain was ahead in things like Radar, Naval Guns, AA Guns, and Torpedos. America WOULD catch up but it would be a problem for the first ships rolled out post outbreak of the war (First two years) in which while America would produce MORE Britain would produce better.
Diplomacy - Britain would of course bring in all of the Commonwealth, I do not see a situation where Australia, NZ, Canada, South Africa and the Raj do not join the war. Britain might also receive the assistance from the Japanese, and from Portugal. Portugal having a similar impact to that of New Zealand/Australia. If Japan were to join on the British side then they would tie up considerable American Navy Assets in the pacific that would result in America being out numbered for the early-mid period of the war.
My Guess at an Approximate Timeline - Assuming no external intervention bar the Japanese
War is Declared. Outbreak 1939 - America declare war on Britain, after Britain boards an American ship in the Atlantic to capture an Irish Free State terrorist. The American merchant ship sends out a distress call, an American warship responds resulting in a naval skirmish in which the American warship is sunk by a British torpedo.
Canada successfully burns nearby cities, and razes industry before retreating ahead of the American Army.
Canada digs in and holds despite heavy fighting for a number of months (Six or so - time for Britain to send some assistance along with Australia, and NZ)
Royal Navy begins to move into the Atlantic, America moves to oppose them lacking the Pacific Fleet due to fears of Japanese Intervention. The Royal Navy have moderate success in an Atlantic naval battle, keeping sea lines open to supply and reinforce Canada and allowing the Royal Navy to raid the American coast.
1941 - Fall of Canada despite early success holding the American army eventually mobilization of the Americans managed to beat Canada. Due to Naval superiority most of the B.E.F moved onto New Foundland which remains under British control as an American landing is not tenable due to losses. Canadian cities are severely damaged but put up continued resistance against American occupation.
After 2 years of this type of warfare, American production catches up and Britain starts to pull back. British support for the war is high and American support is low. British ships are superior but production is massively out paced. America attempts to knock out the Asian colonies by striking at Singapore, Australia and New Zealand.
Japan which is at war with China and running out of Oil, sees that America is moving in the Pacific a combination of paranoia and opportunism sees Japan strike at the American Pacific fleet this is catastrophic for America and sees them pushed out of the Island bases. Invasion of Phillipines occur.
Stalemate continues into 1942/43 - British technological advantage is now nullified in all areas except Nuclear and Jet Aircraft. Gloster Meteor makes it first flight in march 1943 as it did historically far ahead of the American efforts to develop a jet fighter.
1944 - Smaller industrial capacity but with superior land based aircraft based in Newfoundland along with equal/slightly superior carrier aircraft allow Britain to retain air superiority over Newfoundland and Naval Superiority in the Atlantic. Priority put into creating a naval version of the Meteor succeeds in late 1944 production begins immediately. America is a year or two away from jet aircraft.
1945 - Britain develops Nuclear Weapons in Newfoundland (six months ahead of America) and builds a single bomb. This would place the empire in a terrible position debt wise - (The cost of doing this alone was prohibitive which was the main reason that Britain chose to join the manhattan project. However British contribution made the manhattan project actually succeed - It's my view Britain would have beaten America to the bomb but it would have been incredibly costly and effectively bankrupt the Empire after the war.) A single nuclear weapon is dropped on New York, Britain informs America to leave Canada, and attempts to bring them to the negotiation table under the guise that they have more Nuclear Weapons and will target Washington, Philadelphia, and other major eastern seaboard cities if America does not negotiate. The American president is informed that while it is unlikely Britain has any more nuclear weapons, if in six months America does successfully test a nuclear weapon it would have no real way of successfully getting it onto Britain, due to British air defence and radar stations.
America surrenders conditionally, to Britain.
End result - America pays Britain war reperations, and is put under a washington esque treaty limiting its carrier fleet. America is forced to pay for as much of the war as possible. The Philippines and it's pacific islands go to Japan apart from those closest to Australia and New Zealand. The Canadian and American border remains the same. Puerto Rico is passed to Britain as is the Panama Canal. Most of the result of the war is in the economic treaty. That manages to ensure the Empire doesn't collapse, Britain itself is untouched by the war meaning the money recieved would keep the Empire strong.
Comments: It's down to who achieves a number of things 'first' when its Britain vs America. Radar, Jet Aircraft and Nuclear Weapons. Not to mention ship design, British ship design has always been Atlantic focussed while America saw the Pacific as its main theatre for naval operations this meant that during the early war 1939-41 Britain would have success in the Atlantic allowing Canada to hold out longer, and provide dogged resistance and keeping Newfoundland free from invasion. 41-43 is the great stalemate. Britain is not longer strong enough to strike out at the American coast due to superior American ship production but is able to fight in the Atlantic with a great deal of success not to mention it's superior starting technology manifests itself as a slight advantage even at this stage. 44-45 is the end of the war and the period of experimental weapons. Britain historically beat America to military jet aircraft, and was far ahead of America in developing nuclear weapons. In my view leading Britain to a situation in which it knows it will lose the war in 46-48 if it remains conventional but by using a nuclear bomb it can bring America to the negotiating table. If Japan did not join in 1941 I don't think Britain would be able to hold onto Newfoundland or keep the Atlantic stalemate going as advantageously as I describe, and the end result would be similar but also involve a more damaging war for Britain. Post War: Britain would be booming (no blitz) but would be full of debt - this would be payed for by the Americans in some way due to the treaty. Britain and Japan would probably end up creating an Anti-American treaty, Canada would have a massive enmity toward America and would be more Pro-British.
To elaborate, the Seafire suffered from lots of problems most particularly early in its career, which i assumed would be at the hight of the conflict. At later dates, especially if we add the Sea Fury into the mix, they'd be going up against Hellcats, Corsairs and Bearcats (with the jet age just around the corner). With the US numerical advantage, i dont see how that could end well.
