Britain had the largest fleet in the world during that period with France close with the second largest.

The treaties that were put in place to limit capital ship size were bypassed by the many loop holes within them, hence how the North Carolina and Nelson Class battleships could be constructed. Britain also had much more experience fighting naval battles than the US, which would have worked in their favour.

Not to mention the large amount of US naval forces that were moored at Hawaii that would need to either sail around the American continent or through the Panama Canal. Which would have been blockaded and/or mined/destroyed.

Historically, the rainbow plans were made for use against every major power. After all, you can can tell what the future will hold.

As far as Plan red goes, initially it would actually have been a chaotic war, as both Canada and the US had small armies of less than 10 divisions each. Also, as most of the armies were infantry and slow moving, there would have been little combat as the US/Canadian border was huge. The US Navy, due to the shorter distance and thus closer supply lines, would have been in a better position not to blockade but rather interdict the sea lanes between UK and Canada, making it harder to get troops and more important to supplies to Canada. UKs 1930 BB fleet may have been on a par with US fleet roughly 12 vs 12 (number changing slightly as they were scrapped) but battle damaged UK ships would be at a huge disadvantage and the oil needing to be transported across the ocean would hurt.

The most telling difference would be the activation of the National Guard. The guard served as a cadre for the US army in WWII to build around and allowed the larger population of the US to build up divisions with trained leaders quickly. Thus Canada and whatever UK forces would initially be able to invade to US and take cites (as the US would also do to them) but as there were not enough mechanized forces to make huge gains (like hitler would in later years) time would work against them as their infantry would meet increasingly steep resistance. Also, because of the population difference, the UK would have eventually failed as they would be unable to replace decimated infantry divisions (as at the end of WWII, when UK was forced to close units and combine them)

With Canada being such a large country, even though the US would eventually be "winning" by taking large concentration of land, they would never be able to occupy it all. Certainly all of the cities would be gone within two years as there just aren't that many, but the countryside and open border would create a HUGE partisian problem. I agree with Stormvanger that a US victory would be inevitable as the huge manufactering and production base was much greater, but the obsticales of a partisian war followed by a battle for the atlantic, then an invasion of UK would be a logistical nightmare and probably worse than operation overlord, as there were no bases to invade from.

Overall, just to make the record clear:

3 Midway Class carriers
24 Essex class carriers
3 Yorktown class carriers
2 Lexington class carriers
Wasp
Ranger
10 Independance class carriers

6 Iowa class BBs (the last two were nearly complete, not just paper plans like Lion)
3 Alaskas (One was nearly complete)
12 Standard BBs (New Mexico, Arizona...)
4 South Dakotas
2 North Carolinas
14 Baltimores
28 clevelands
18 New Orleans/Northampton/Portland/SLcity
9 brooklyns

vs

Aircraft carriers
2 Glourious and Furious
Eagle
Hermes
Ark Royal
Unicorn
4 Illustrious
2 Implaccable
10 Collususs
6 Majestics

BBs
5 King George V
2 Renowns
Hood
Vanguard
5 Resolutions
5 QEs
72 6" and 8" cruisers

Not only were the UK ships outnumbered but also outgunned. Also, their CVs had armored decks but only sported 35-48 planes as opposed to 70-90 (for the fleet carriers). In terms of aircraft, the UK could bring 1100 planes to fight as opposed to about 3000 for the US. Another killer to this equation was the firepower difference of the BBs. US Iowas, Sodaks, NC sported 90 x 16" guns on modern platforms. Where as UK had 55 x 14" and 16x15" guns on modern platforms.

So early in the war, the UK would have an even fight against the US, but as the war went on, the US would have a lot more bigger, faster, more powerful ships, especially planes. UK would end in a similar situation to Japan.

Pax Britannica people.. only interrupted by the first world war.. which only served to make the Royal Navy stronger.

I stopped reading Lionel2s post after i glanced down and saw those ship numbers.

lionel i am not going to pick you up on ship numbers as others have beat me to that punch, however i will pick you up on this................

the UK would have eventually failed as they would be unable to replace decimated infantry divisions (as at the end of WWII, when UK was forced to close units and combine them)

the only reason at the end of WW2 the units were closed/combined was NOT the lack of man power but the lack of money as we had to rebuild our citys and inferstrucer due to bombing, which the americans did not have to worry about.

so you could turn it on its head and say that if this plan had been actioned, then detroit would not have become motor city for sure it would have looked like dresden. so in the late 30's and 40's america would still have been rebuilding, while the UK out of harms way would still have been building and expanding its air and naval forces.

Not to mention how easy it would be to kill huge numbers of American civilians in comparison to England, Germany and even Japan thanks to the compactness of Americas towns and numbers of skyscrapers within them.

Britain would not only have had troops from Canada Scotland, Wales and England, but from India, Australia, New Zealand, parts of China, South Africa, Egypt, Pakistan, Kenya, Uganda, Nigeria, Sudan and the many other African countries under British rule. As well as the much feared Gurkhas.

Add on top of that the French, Belgian and Dutch armies and navies and not only would naval strength be on Britains side but land based forces as well (obviously once they landed on the American coast or Canadian coast). The key coastal regions of America would have been taken or shelled and eventually force what US forces that were there into retreat.