Who thought there might be a war between the British Empire and the United States of America in the 1920s?
There was no naval arms race in the 20's, on the contrary both Britain and the United States would enter into parity agreement which would call for significant naval draw downs from 1922 - 1937.
Nobody!. Well, Nobody without an agenda, thought their would be a war between the United States and Great Britain. There were a few people on both sides of the Atlantic talking up the threat in an unsuccessful attempt to get more funding for their own Navy's, both of which were on the precipice of a large draw down from the political majorities of both countries. Great Britain politically powerful Navy was not happy about accepting parity and massive draw down. The United States Navy also had it's supporters talking up the need for American to fully fund parity with the British Navy. Both efforts were unsuccessful. Great Britain, certainly did not desire a war with the United States. Great Britain was weary of war, and her politicians were unwilling to support the existing British Navy much less finance an arms race. The United States leadership balked at funds even approaching parity with what the British were willing to spend. The United States and United Kingdom had just concluded WWI(4/6/1917 - 11/11/1918) where they were close allies against an existential enemy. The friendly governmental relationships between the two nations paled only in comparison to the close cultural historic and business relationships of the two peoples. For the years (1900, 1913, 1928, 1935, 1938, the UK was the United States largest trading partner. More than half of all US annual Exports during the interwar period went to the UK see Table XXIV - 1 ) The US banking system and that of the United Kingdom were tied together with many hundreds of millions of dollars in loans.
There was no naval race in the 1920's. As I said before, World War I ended in November 1918 and all the great sea faring nations had large Wartime Navy's. Great Britain owned half the dread naughts on the sea at the end of WWI. The challenge wasn't to build larger Navy's, the challenge was to disarm in an organized way to ensure stability. Before an arms race could occure, the five greatest sea faring nations got together to ensure their would be no "possibility of an Naval arms race". The United States Secretary of State (Charles Evans Hughes) calling all the major Naval powers to the(Conference of Washington 1921-1922)
- United States
They negotiated (the five nations treaty, 1922) to organize a Naval draw down.
The Terms of that Treaty were extended at the London Treaty of 1930 which stayed in effect until Dec 31, 1936.
The fact that Great Britain the most powerful Naval power for centuries at the time accepted parity on paper in 1922 with the United States demonstrates that Great Britain didn't think the likelihood of a war with the US was significant. The fact that by treaty the United States could build a Navy equal with that of Great Britain but chose not too, demonstrates there was no concern on the US side of impending hostilities from Great Britain.
Twisting the Lion's Tail: Anglophobia in the United States 1921-1948
But the call for increased naval spending ran into powerful opposition in the form of President Coolidge, who in February 1927 proposed calling a conference which might extend the Washington ratios to cruisers and auxiliary vessels. Though personally no less committed to naval parity than his predecessor, Coolidge balked at the size of the government expenditure which would be required to build up to England's level.
The Naval build up / arms race didn't begin until 1937 and then it wasn't the British in competition with the US.
The British were not concerned with the possibility of a war with the United States during the interwar period.
British Naval Policy - 1920-1939.
Naval rearmament was limited from the outset by the disarmament
process. Expansion only got underway after 1936, once the limitations
of the 1922 Washington Treaty and 1930 London Treaty expired on 31
December 1936. The Defence Requirements sub-Committee's standard fleet
(their projection of what the Royal Navy would need to meet certain
commitments) essentially brought the one-power standard (which stated
that Britain's navy should be equal in size to the biggest naval
power) up to date. This involved the modernisation of existing
warships to compensate for the deterioration in qualitative superiority since 1922.
Winston Churchill, the member from Epping, told the Commons on 16 March 1936 "The foundation of British naval policy is the acceptance of the principle of parity with the United States of America, not only in battleships but over the whole range of the Fleet. We are all agreed upon that, and that decision once taken ought to exclude the idea of naval rivalry between the two countries. It certainly ought not to be followed by a meticulous measuring of swords, as it were, at recurring conference tables. The British view is, and has long been, that the, United States Navy, whatever it rely become, is no cause of anxiety to us. On the contrary, many people will feel, and it is no exaggeration to say so, that the stronger the United States Navy becomes, the surer are the foundations of peace throughout the world. I trust, therefore, that the principle of parity which is really the principle of non-competition, will be interpreted in the most liberal and flexible manner on both sides of the Atlantic, and that the two great branches of the English-speaking peoples will not seek to hamper one another in making whatever may be the best possible arrangements for their respective naval defence.
With Regards to Plan Red
The United States had drawn up a detailed war plan envisioning a US Invasion of Canada followed by a US invasion of Britain in the early interwar periods. This may be the basis of your question; however,
the United States has a long history of writing and maintaining unthinkable war plans. The United States Military today maintains plans to confront a Zombie Apocalypse, and an Invasion of extraterrestrial Aliens.
“CONOP 8888.” It’s a zombie survival plan, a how-to guide for military planners trying to isolate the threat from a menu of the undead — from chicken zombies to vegetarian zombies and even “evil magic zombies” — and destroy them.
If that seems comical to you it is not. These far fetched war plans serve a real purpose. The United States Military is a large complex bureaucracy. To effectively wield such a monstrosity takes practice. What practice means is the United States has war planners write a lot of plans nobody ever expects to use. Those plans are then critiqued, discussed, graded, and revised. That is how inexperienced peace time war planners become experienced planners.
Today these interwar plans are known as the Color-Coded war plans.
After WWI the United States military developed 150 various war plans. To highlight how absurd the plans were, not just because the relatively small US Army in the interwar periods were keeping such extensive plans.. but that they did not keep their plans updated for the most likely antagonist, the power with whom they had just fought WWI. Germany.
United States color-coded war plans.
Many of the war plans were extremely unlikely given the state of
international relations in the 1920s, and were entirely in keeping
with the military planning of other nation-states. Often, junior
military officers were given the task of updating each plan to keep
them trained and busy (especially in the case of War Plan Crimson, the
invasion of Canada). Some of the war plan colors were revised over
time, possibly resulting in confusion.
Interestingly, although the US had fought its most recent war against Germany and would fight another within twenty
years, intense domestic pressure emerged for the Army to halt when it
became known that the Army was constructing a plan for a war with
Germany; isolationists opposed any consideration of involvement in a
future European conflict. This may have encouraged the Army to focus
on more speculative scenarios for planning exercises.
As stated here previously the United States Army when WWII began July 1939 was smaller than that of Portugal's, and months before Pearl Harbor when every informed person in the country thought war was imminent, the US army survived being decimated by 2/3rds by a single congressional vote. The US military during the interwar period was not maintained to be a threat to any European power, much less the first tier powers such as the UK. The US military didn't have the political support, financial support nor the intent to take on such a challenge. So why did the United States have a plan then? PRACTICE!!... They were training future war planners and they did so by taking every imaginable and many unimaginable scenarios and mapping them out and then reviewing and critiquing and refining. Just like we do today.
As you can imagine when these secret plans were eventually released to the public. All the Color-Coded plans, including Code Red (Britain / Canada invasion plan) from the interwar period released in the 1970s; considerable angst was generated from close allies. Which brings us back to the Zombies and extraterestrials Aliens. Examples of politically correct "enemies" who the US military can use as learning scenarios without giving our closest allies and half the world heart attacks.