This is far and away the most intelligent comment! Reading the OP, it's completely ASB so ending it in "just keep it plausible" is particularly ridiculous.
Anything that can
plausibly bring the US to blows with the British Empire in the time frame "Plan Red" existed (1927, we are told, being when it was drafted, WWII being when it became a complete dead letter, so 1927-1940) will of course change the plan. If a sequence of events can so dramatically reverse the prevailing tone of good relations that among other things, had left the huge US/Canadian border unfortified on both sides for a century, it must be having dramatic effects on both countries long before hostilities actually start.
Now there was of course a dramatic, life-changing event that happened not long after Plan Red was drawn up--the Great Depression. And the different reactions to the Depression (not to mention the blame laid on American institutions or the lack thereof for the catastrophe) on the opposite sides of the Atlantic were a point of controversy. It was in fact mobilization for WWII that brought the lingering effects of the Crash to an end at last, in all countries involved. Perhaps something could be done with arranging a cascade of events that leads to such a war in the wake of the stock market crash of '29. If the actual war starts in say 1931, then there won't be much time for a massive buildup or mobilization of new forces and plans drafted in 1927 might still have some relevance.
A few things seem to have been overlooked thus far. If the two powers are going to war in the early 1930s, they both still have a lot of veterans of WWI to draw on; they are out of training and it was obsolete in any case, but on both sides the pool of potential recruits who could be brought up to speed fairly quickly and indeed could be put in command over truly fresh new young blood is rather large.
I was even wondering if this could avert the whole Veteran's March on Washington of OTL; with the country going on a war footing the veterans could be given their bonus, on the condition they report for re-enlistment--the ones the Army doesn't want for reasons of health or being overage still get it, as do the re-enlistees, who are put into ranks equivalent to the highest they held in the last war or higher after some quick re-training.
Such a war might be just the thing Herbert Hoover needs to be re-elected!
There's also Mexico to consider, and Latin America in general and the existing US presence in the Caribbean in particular. We in effect ran Cuba and Haiti in addition to our outright possessions; I'd expect the regimes in Havana and Port Au Prince to sign on as US allies pretty much regardless of the sober and rational interests of their peoples. If Washington can daunt the Dominican Republic as well the whole string of the Greater Antilles is a barrier for the RN, with the USN haunting the straits between them and US land forces propping up the regimes there.
But Mexico---can the British guarantee the Mexicans lavish supplies and subsidies and a credible defense of Mexico if things go badly, to entice Mexico into the war against the US?
Vice versa, all Washington needs out of Mexico is friendly neutrality--will the Hoover, or perhaps early Roosevelt, Administration have credibility when they assure Mexico they won't suffer and will benefit from staying friendly to the USA? Surely no American government needs to spell out to Mexico that if they are not friendly, their allies had better be triumphant because the US will react badly to attacks from the south.
After all, even if this war does go very well for a British/Mexican alliance and the result is Mexico regaining substantial territories, Mexico will have to strenuously guard her new northward borders for generations to come, unless the USA is crushed or shattered out of existance.
So Mexico might well sit it out, but a detailed situation arising from a specific POD might make an active British alliance less implausible.
Let's not forget this time period is also the time when a fair investment was made in LTA by the US, which had the only helium supplies then known in the world. In addition to the Zeppelin-constructed USS
Los Angeles, the USN in the timeframe 1927-34 operated at any given time a succession of three other rigid airships made in the USA; in 1931 the LA was still in active service and was joined by the USS
Akron, then the largest airship yet made. And the Akron operated a small squadron of small scout-fighters. Besides this, both the Navy and Army operated blimps of increasingly sophisticated design. In OTL WWII the blimps became the most numerous fleet of airships ever operated by any entity, and served to greatly deter and suppress submarine activity on the American coasts, as well as a number of other functions.
A blimp patrolling the coasts would of course be dead meat at the hands of even the biplane fighters typical of air craft carrier service at the time, but it would serve its purpose of alerting the USN of a carrier present and its rough location. Aside from carrier (and flying boat) attacks, they'd tend to force British submarines down while American subs would have free rein.
I'd expect that the US submarines would be quite a force multiplier in the friendly waters of the US coasts, greatly inhibiting RN freedom of action and again where American subs dominated, giving the USN the initiative.
It will not be so easy for the RN to terrorize the American coasts as some here have been assuming, not at any rate in the post-WWI era. This buys the US time to build up truly massive forces, on land to overwhelm Canada (and if need be, hold a British-backed attack from Mexico at bay then retaliate with a deep invasion) and in ships to eventually break out, having first decimated the RN if it tries to push through to US shores.
Whatever might happen to our holdings overseas, I can't see the British threatening to break the bastion of the lower 48 states.
It might not be strictly true the USA was fully autarkic, or even could be; certain strategic materials might only be replaced by quite inferior substitutes. But any time after WWI (until much later, when we had vastly outgrown our domestic resources) I'm sure we could at least limp along with rubber substitutes, somewhat inferior alloys, and so forth, using vast (by the time's standards) oil reserves and a highly advanced chemical/metals industry to develop them.
If the USN can break the noose of the RN (and I think that's more a matter of when than if, especially given our developing aeronautical abilities) the Americans could conceivably find trade partners in South America who might have the metals and other resources--rubber especially--we would need, and might, if we can demostrate such abilities, risk getting on the wrong side of the British in return for the right considerations. We might perhaps never get back the Philippines, might lose even Hawaii, but if we lose the Canal Zone I think we could make it useless by blockading both ends and indeed besieging any British forces in Latin America; the upshot would be, if we don't break Britain completely by sinking all RN forces sent against us then breaking out and savaging British commerce everywhere, we could at least secure supply lines holding all of Central America, the Caribbean, and eventually South America in our sphere and excluding Britain and anyone allied with her. Holding the Western Hemisphere I do think an American hegemony could be autarkic. Also we'd gravely threaten European hegemony over western Africa.
Our downfall of course would be if we failed to secure control over Latin America by alienating the Latin Americans; chances are pretty good we'd screw that up.
By the way I'd think a war like this would
be WWII. We'd completely butterfly all the major combatants of the OTL war. Japan, I believe, would indeed ally with Britain and gain from it; the British would among other things give their blessing to Japanese ambitions in China, demanding only that Britain be cut in on the spoils, but compensating for these by sharing markets as well as resources with their East Asian allies and assisting in Japanese development technologically. So China might still be a bloody mess but until and unless the Americans get to the point where they are seriously sapping the Empire's vitality, it won't be called a front of a global war and the Chinese, for the most part despairing of any help (except perhaps from the Russians) will come to terms with Anglo-Japanese domination. (The Communists might be exceptions, but OTL the most tenacious were those who defied Moscow, like Mao, rather than obeyed Stalin--we don't remember the names of Mao's rivals because Stalin's advice was not good for their health.

It raises the interesting spectre, if we have an angry USA on the warpath in the Pacific, of Mao getting most of his aid from the Yankees.

) So with a good chunk of China and access to British Empire markets for other goods (and through British good offices, or indirectly through British markets, the goods and markets of other European colonial powers too) the Japanese will have little need to push for more, especially if they have also been able to gobble up the Philippines with British blessing.
In the west of Eurasia--Hitler's rise to power is probably butterflied away if this happens before the end of 1932. Weimar Germany will probably benefit from British wartime demand, holding the worst wolves of the radical degeneration of Weimar in the wake of the Depression at bay. The usual political seesaw between the moderate-conservative parties and the Social Democrats would probably be able to continue in much less emergency conditions than developed and worsened OTL. Hitler's party won't be able to come even as close to a majority as it did OTL (it never did become one in the Reichstag, and the last election before Hitler was made Chancellor by Hindenburg's emergency powers decree they'd lost ground from the one before). There will be no need for the conservative power brokers to bring him in. If the war puts enough of a strain on Britain the British will possibly advocate for Germany to be relieved of its restrictions on arms manufacture, provided they then sell those arms to Britain. (Possibly the British will still be in a position to assert Germany still owes reparations--but will take those reparations in the form of the most advanced munitions and airplanes and so on German industry can offer.) Germany could be degrees be drawn into the war on the British side. But I don't see the rise of a Germany as fanatically devoted to conquest for power and vengeance as OTL. Possibly one determined to take back the Polish Corridor, and take hegemony over Poland, possibly to try to dominate Czechoslovakia as well and to have a free hand in Eastern Europe in general. But not one that threatens to refight WWI with the western Entente powers all over again, nor one that would feel itself in a position to offer to ally with the Americans--not unless the Americans were so overwhelmingly victorious they were invading the British Isles themselves.
Even in that unlikely event, we clearly don't have a prelude to a different World War II, we have that war itself a decade early, and with different combatants.
Obviously, in such a war, which I still think is quite ASB in its premises, my money is on an ultimate Yankee victory, one as sweeping and global as the American care to fight for. If we can wind up dominating the whole Western Hemisphere and keep it (without blowing it politically I mean) we might well choose to stop there, forgetting about vengeance for the Philippines and so forth (until it is convenient anyway

.)
Equally obviously, it is silly because both powers lose more than they gain. WWII was generally a war of the have-not Axis against the mostly "have" Anglo-Americans; in their own paranoid and envious way, the Soviets too could be characterized as a "satiated" power in the sense that they had no serious plans to expand their borders by force and were resolved to progress in terms of what resources they had in the "socialist motherland" and their later expansions were strictly opportunistic (and in only partial compensation of devastation they had suffered).
There was just no compelling reason for either side to risk war with the other, when even the most serious differences could be amicably compromised or worked around. The costs were too high, the benefits too dubious and metaphysical.
So to return to the OP--you can't say "never mind how and why this happens, just suppose it does!" You have to at the very least say
when it happens so we have some sense of what technology each side has, what their basic needs and vulnerabilities are. And just saying "Go!" is quite ASB. I sort of implied that perhaps the crisis of the Depression could make for sufficient crabbiness, but I don't really believe it; setting Americans against Britons to the death like this requires some sort of ASB mind control, otherwise you have to identify how this conflict develops over years, consider the likely steps and counter-steps each side takes over the gradual brewing of the crisis, consider who in each nation supports the war and who is against it, because these determine how much will each side has, how much loss they will endure before seeking a truce. OTL the answer on both sides was "none, no bloodshed!" and the chances that the few cranks on either who might have pushed it farther could achieve that seem vanishingly small to me.
So I've taken this whole thread in an ASB spirit, just trying to address the question of what would happen if both sides jumped into it wholeheartedly until one or the other breaks.
Under those conditions I believe the USA is unbreakable by post-WWI.