Let’s not dispute the senario.
Okay then, we'll stick to the scenario.
Allies: US, USSR, France
Axis: Germany, Italy
Imperials: UK, Japan
(Nationalist) China might work with either Axis or Allies, depending on whether the US or Germany courts it successfully.
Geography is remorseless. The Americans in 1939 have no realistic way of striking at the UK homeland. The British absolutely can and will do so for the US. The question is not that the USN is a “joke”, it’s not, it’s just that in any war, it’s going to be fighting to keep the RN away from the US Coast, while it’s bases in the North East, as well as the shipyards are going to be major targets for RAF. Rather than having a secure base to rearm, rebuild and refit, as it did in the OTL Pacific War, it will have to expend tremendous resources in Homeland Defence.
Galvanised or not. The massive shipyards and aircraft factories were untouched in OTL. They won’t be in such a war.
Which is why OTL 30’s plans for use in a war against the British empire, US strategy was mostly land based. Force the British **out** of N America.
That's certainly true. But there's one other way they can
destroy the British; sink their commerce. Britain was entirely dependent on its shipping. Once the USN manages to retake the initiative, the British are going to lose a lot of ships. Any shipping is going to go through the Atlantic Ocean.
The other side of the equation, though, depends on how well, the US-Soviet alliance holds up. But first, let's go through how this was will progress, and let's start from, say, 1938.
1938:
By Munich, it's become increasingly clear the British are becoming less and less interested in keeping balance in Europe, despite desperate French entreaties. As much as they plead with their former Entente Cordial friends, London does not budge. "The Sudentenland," says the British PM, "is a German matter, and Germany has every right to reclaim it." Paris is in an uproar, furious at the callous disregard for the weakening peace in Europe. Unbeknownst to most of Europe, the British and the Germans have signed a non-aggression pact; Britain is to turn a blind eye to German actions, while the British Empire is to be free of German attempts to invade or influence it. Both sides have their eyes on bigger fish; the USA and USSR. Nazi hate for Socialism is no secret, but it's become clear in the past decade that British contempt for the 'colonials' has gone on the rise. The USA has been excessively demanding repayments on its WW1 loans during the depression, while denouncing British acts in its colonies. France has staved off American demands by vague promises to improve things and try to speed up decolonization, and fostered a closer Washington-Paris relationship. As a result, Britain is convinced that the USA is its new existential foe. Many pro-US politicians, such as Winston Churchill, son of an American woman, fall out of favor of 10 Downing Street and effectively "sent to Coventry".
The USSR tries to offer the French a secret deal to take down Germany before it takes the Sudetenland, offering to help Poland bolster Czechoslovakia and prevent the German invasion, but Polish obstinacy and fear of Soviet infiltration ruins the chance. With the Polish secretly working with the Germans to take claimed territory from the Czechs, the Polish want to stay neutral. This ruins the French opportunity, forcing Paris to accept the concession, thinking it can't fight a new war against Germany without suffering losses similar to WW1. The French, as such, start rearming and building fortifications, with Paris calling for 'Peace in Our Time'. Britain applauds the event, but acts as if nothing happened when the Germans annex the rest of Czechia and turn Slovakia into a puppet state, with Poland taking Galicia. The countdown begins.
Japan has already invaded China in defiance of the League of Nations, with Germany, USA, and France condemning the move, but Britain defending Japan as 'defending its interests against vandals'. With the Anglo-Japanese alliance still strong, Japan continues its invasion of the divided and weak Nationalist China, much to America's anger, and the rhetoric increases. In response to deteriorating Anglo-American relations, the US-Canadian border becomes increasingly fortified and militarized, with talk increasing in the US about restarting the draft to counter any possible threat from the north. It is defeated in the Senate, but the US Army slowly begins its preparations. The USN also starts improving within its budget. Britain, meanwhile, starts increasing its military presence in Canada, ostensibly in response to 'American adventurism and aggression', and sets up
Defense Scheme No. 1 in preparation, while the Americans secretly set up War Plan Red.
1939:
Germany, having built up some of its forces in preparation for the invasion of Poland, invents a
casus belli against the Poles, demanding Danzing and the Polish corridor. Already having guaranteed the USSR's neutrality with the Molotov-Ribbentrop Pact, it threatens Warsaw with war. Paris demands Berlin stand down and cease its aggression, calling for an intervention, but London disregards it all. Again. Poland is invaded, France declares war, and things go from bad to worse as Poland is invaded from two sides and then partitioned by Germany and the USSR. France mobilizes and prepares for an all-out war. Belgium, believing itself safe with French and British guarantees of its independence, nonetheless starts building up its defenses. The "Phony War" starts in December, where the French and Germans stand behind fortified borders and wait for the other to make the next move, while the USSR bungles into the Winter War, exposing the weaknesses of the Red Army.
1940:
After a Phony War where Germany builds up its forces, its steel imports from Sweden guaranteed by British neutrality, it then turns its eyes on France and the Low Counties. Denmark is taken over with minimal resistance, the Low Countries fall despite their preparations, and I will assume the Invasion of France goes as it does OTL, only without the BEF's help. Paris falls, Europe is now effectively in Nazi hands. The USSR begins arming up immediately, as they don't have the luxury of waiting for Germany to deal with Britain and know they're next. Many in the CPSU still hope Germany and Britain will turn on each other.
Here, it depends on Italian actions. Without British interference, they still get humiliated in Greece, but it really all depends on whether Italy's invasion of Greece will trigger British intervention, or if they'll just stay neutral in Europe and focus elsewhere. Instead of taking Egypt, Hitler tries to talk Mussolini into taking Algeria and Morocco instead, as he doesn't want to anger Britain just yet. Not until he's dealt with the bear. The Balkans fall to Axis control, but Italy's bumbling ensures Barbarossa is delayed by several weeks, moving it to late summer, 1941. The USSR, meanwhile, is still trying to convert its lumbering mass army into a mobile striking force, but officer purges and the sheer mass of the Soviet Army makes things difficult. Germany makes overtures to Finland to get its revenge.
Britain continues preparations for war, and Japan, meanwhile, takes advantage of the French collapse to take over Indochina and a few possessions in the West. Britain and Japan split Siam between them, effectively cutting all roads of supply to China. China is going to have a much,
much worse time in the coming years.
The US high command debates launching War Plan Red, but eventually decide against it despite some warning voices, opting to work on their defenses more and on the USN. That, as they will discover, was a big mistake.
1941:
With all its ducks in a row, the Axis forces storm the USSR in the biggest land invasion since the Mongol expansion, overtaking the still-underprepared Soviets across all fronts, though the Southern front has a harder time than the North or Central fronts due to having weaker Axis forces, like the Romanians, Hungarians, and so on, facing the better Soviet units defending resource-rich Ukraine. Later in the year, the USA is overwhelmed by massive numbers of British, Canadian, and ANZAC forces pouring across the borders, supplemented with Indian troops for numbers. Initial defenses hold out in some place, but are outflanked and soon overwhelmed, while the British charge into New York, New Jersey, and the Great Lakes, while the RN and RCN fight the USN in the Atlantic, and the IJN and ANZAC ships engage them in the Pacific. IJA troops overrun the Dutch West Indies, secure from the British in Singapore and India.
From here on out, it depends on several factors. If Italy doesn't foolishly charge into Egypt and incur Britain's wrath, the Imperials and the Axis can safely ignore each other, though Musso will definitely whine about wanting the wealth of Egypt and the Middle East into his new Roman Empire. If he does, though, the Axis and Imperials come to blows, forcing both Germany and Britain to delay their plans and give the US and USSR more time to prepare. It would be ironic if DS1 and Barbarossa both get delayed because the two nations had to fight each other first.
If London, Berlin, and Rome remain on good terms, however, the invasions proceed as planned. The USSR starts massive conscription, and the USA tosses caution to the wind and starts a frenzy of conscripting and drafting troops just to protect what was left. FDR and his cabinet move to safer areas down south, California, the MidWest, and the South start fortifying, and the US tries to move its industries down south and into California to keep them away from the bulk of the British invasion, much like the USSR is doing moving its industries past the Urals and fortifying Leningrad and Moscow. Though without a friendly Iran and with the RN blocking the seas, the US won't be able to send anything to the USSR - not that it would, anyways, given that they're also under attack and trying to fend for themselves.
Eventually, the US and Soviet governments sign a pact with Free France to form an alliance, and vow to fight back against the Axis and Imperial aggressors.