The US military, like many militaries, has various plans for nearly any possible conflict. These plans have included how to handle a conflict with the British Empire as it was one of the few powers that was seen as a plausible threat. These serve plans serve two purposes, to practice planning (often in some of these conflict plans this is the primary goal) and to have these plans just in case something occurs. The British had one of the most powerful militaries and given that 1: Canada shares the longest international border in the world with the US thus is one of only two land routes into the US (Mexico being the other) and 2: The British had the naval and commercial capacity to launch a large scale attack on the US which few states have ever had up to this day.

Under the Rainbow plans military planners had drawn strategies for hypothetical wars with:

Canada -Crimson

Britain-Red

Newfoundland-Red

India-Ruby

Australia-Scarlet

New Zealand-Garnet

Ireland-Emerald

Any part of the Empire not covered (Africa and Middle East) - Pink

Germany-Black

Japan- Orange

ETC.

The US was not the only one planning for such a conflict. The Canadian had Defence Scheme No. 1, the Canadian defense plan to defend against any possible war with the United States that included provisions for a preemptive surprise attack on the US. Canadian officers had even conducted reconnaissance for the plan (including its author Lieutenant Colonel James "Buster" Sutherland Brown) The British had an informal plan of their own as David Lloyd George in had place restriction on any formal strategic planning regarding a war with the US in 1919. The British deemed in unwise to plan for a land war or blockade, instead opting for limited naval base conflict and hope for a negotiated peace.

To illustrate how broadly the US military plans it has plans for a zombie outbreak (see oreignpolicy.com/2014/05/13/exclusive-the-pentagon-has-a-plan-to-stop-the-zombie-apocalypse-seriously/) and rumor have persisted about planetary defense plans from extraterrestrial threat (though none have ever official surface)

Preston, Richard A. The Defence of the Undefended Border: Planning for War in North America 1867-1939. Montreal and London: McGill-Queen's University Press, 1977.

Bell, Christopher M. (November 1997). "Thinking the Unthinkable: British and American Naval Strategies for an Anglo-American War, 1918-1931". The International History Review. Vol. 19, No. 4 (Nov., 1997)

The interwar period was interesting.

First we should recognize that while allied in WW1 and in general sharing many interests, the US-UK relationship was not yet uniquely special, or well established.

It was only really in the 1890's that relations warmed up from being frosty, and even then it was in part the UK seeing the US as a more preferable to say the German's getting into the empire game.

And with the destruction of the HSF the USN was really the single greatest threat and possible foe to the RN, and American ascendancy to a global maritime power was by no means assured to be easy or uncontested. The Officer Class of both fleets got on well, but there were a good number who were less than fans of the other side, Ernest King the CNO in WW2 was famous for his less than warm opinion of the British for instance. Meaning while both sides had grown close in WW1, there was still that undercurrent of tension, and changes in government could always bring changes in policy, and a plan you never use is always preferable to not having one you need.

Second is the fact that Britain, at the time immediately following WW1, was still a treaty ally with Japan. Who after defeating Russia in 1905 was absolutely seeking to be the dominant naval power in the Western Pacific and freeze out the US. In fact in the early 20's the treaty was let to fall by the wayside specifically because the US was a more desirable ally at the time than Japan.

So while the US-UK Special Relationship was growing between the wars. Those on the planning boards and war college staff very much could remember a time when that was not the case. That combined with the desire to have a plan for every situation makes it very understandable to look at the logistics and factors of fighting the British Empire in whole or in part.

Even today the DoD likes to say that there is a plan for every country on earth. because even just going through the motions means the plans you actually need will be better for it.