Complete neutrality. No lend lease, large scale aid, undeclared naval warfare, nothing. The US doesn't do absolutely anything to favor one side or the other and refuses to involve itself in any other countries problems.
PoD for this: Franklin Roosevelt has a heart attack a few days before the 1940 Democratic convention. He doesn't die, but it is clear that he is not fit for another term as President.
It happens while he is traveling across the country; he is treated at a small-town hospital. FDR tries to cover up, claiming it was only a case of mild food poisoning; the chief attending physician gets mad and blows the whistle. FDR is toast for re-election.)
This throws the convention up for grabs.
NOTE: I have never been able to locate any serious contemporary (from 1940) speculation about who might be the candidate if Roosevelt does not run.
The two men who had declared candidacies while FDR had been playing coy were Vice President John "Cactus Jack" Garner, an elderly conservative, and James Farley, who had been FDR's campaign manager and political fixer (DNC chairman, Postmaster General). Neither had developed any serious support.
Another man had been maneuvering behind the scenes for the nomination, in case FDR withdrew: Sen. Burton Wheeler of Montana. He had even lined up a a campaign committee to back his possible announcement. He was the first and best prepared to jump in. Wheeler was an ardent New Dealer, with a long history of liberal activism. In 1920, as Attorney General of Montana, he publicly refused to collaborate in U.S. Attorney General Palmer's "Red Scare" persecution of socialists and labor organizers. In the 1924 Presidential election, he was "Fighting Bob" Lafollette's running mate on the Progressive ticket.
He had also shown that he would stand up to Roosevelt on principle, when he led opposition to FDR's "court-packing" scheme in 1938.
Wheeler opposed FDR in one other area: he shared the isolationist sentiments common in the Great Plains and upper Midwest, and indeed was more isolationist than most. (Though not
all: Wheeler's fellow Montanan, Republican Jeanette Rankin, voted against declaring war on Germany in 1917 and even against Japan in 1941.)
OTL, Wheeler inveighed passionately against Lend-Lease. Alluding to the practice of destroying "surplus crops" under the Agricultural Adjustment Act, he said "The lend-lease-give program is the New Deal's triple-A foreign policy; it wil plow under every fourth American boy" (by getting them killed in foreign wars).
Some in the convention were reluctant to endorse such a controversial figure, but others were attracted by his uncompromising energy. While Garner led on the first ballot, the delegates were deeply split - Garnner had only 209 votes out of 1,093. Farley had only 94. Wheeler had 133. When the convention adjourned after six ballots, Wheeler led with 322. Garner had stalled at 238 votes, and Farley had fallen to 86. The remaining votes were divided among Sen. Millard Tydings, Secretary of State Cordell Hull, former Indiana governor Paul McNutt, House Speaker William Bankhead of Alabama, and some others.
Wheeler had previously offered Farley the VP slot for his support. Farley, seeing that his quest for the nomination was futile, now accepted. He won over several key "city bosses", including Kelly of Chicago and Flynn of the Bronx. Wheeler surged to the nomination over the next two ballots.
The Republicans had nominated businessman and political novice Wendell Willkie. Though Willkie had many strong features, his inexperience hurt him; he made a lot of foolish impromptu statements. Also, he was torn between two forces. The Republican "base" in the Midwest was strongly isolationist. Many "Eastern" Republicans were internationalist, though, and the situation in Europe turned ever more doubtful, with the Battle of Britain followed by the Blitz. Many "Eastern" Democrats were internationalist, and Willkie was urged to take an internationalist stance and win them over - it was thought that this issue would override domestic concerns. Willkie wavered; then allowed some of his surrogates in the east to attack Wheeler's isolationism. This move backfired - alienating Midwesterners without winning Easterners.
Wheeler swept the "Solid South" (though by reduced margins compared to Roosevelt) and the West, and split the Midwest and East, winning with 310 electoral votes to 221 for Willkie.
Wheeler took office in 1941 with an unconditional pledge to keep the U.S. out of war. The U.S. would not aid any other country in any way. U.S. businesses would be strictly prohibited from selling munitions in any form to belligerents, or extending any credit to belligerent countries - their governments or private businesses.
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OK, that sets up the condition.
What are the consequences?
First: it puts a squeeze on Britain from the middle of 1941. Britain's financial resources are exhausted; its demands on the Commonwealth nations and the Empire are intensified.
Second: it limits what Britain can send to the Middle East to fight the North African campaign. This doesn't start to bite until Operation BATTLEAXE in June 1941, or maybe later, but it will really hurt by CRUSADER in November 1941. Most likely, the Axis defeats CRUSADER and captures Tobruk.
Third: if the U.S. will be strictly neutral,
and Japan knows it, Japan is very likely to seize the European colonies in the "Southern Resource Area": Indochina, Malaya, and the East Indies. With Wheeler as President, that would be clear. Some Americans would be upset, but it's very hard to see how Wheeler could be forced to renounce his very strong convictions by events literally on the other side of the world.
Also bear in mind that by late 1941, Japan was running out of money to pay for oil imports from the U.S., and Wheeler would prevent any sales on credit. So Japan would be driven to attacking south.
Fourth: Once Japan has seized the East Indies, it can expand at will into the South Pacific. There is nothing but distance to stop them. Japan cannot invade and conquer Australia - but Japan can send naval task groups to sail all around Australia, bombarding and strafing at will. There are no warships or warplanes to stop them. Australians will be very angry at their abandonment by the Mother Country, despite sacrifices by their troops in the Middle East. It seems probable to me, that to avoid such Japanese attacks, and gain the release of their PoWs (many would be taken in Malaya), Australia and New Zealand would leave the war and declare neutrality. Under Wheeler, the U.S. would never intervene in affairs 10,000 miles away.
Fifth: After the conquest of southeast Asia, Japan could drive the British fleet out of most of the Indian Ocean, practically blockading India. This would probably lead to a collapse of British authority in India, where Gandhi, Nehru, and the Congress Party were calling for resistance. Japan might not bother; OTOH they don't have much else to do with their fleet, and the British are a continuing nuisance. It's possible that Japan moves into the French base at Diego Suarez in Madagascar; the French won't resist, and it puts a stranglehold on the Indian Ocean.
Sixth: All of this will make it much harder for Britain to maintain its sea communications against U-boats and surface raiders. British food shortages will be increasingly acute, especially in 1942, if as OTL the Germans adopt a secure U-boat cipher.
Seventh: None of the above will have much effect on the Eastern Front between BARBAROSSA and late 1942. However, by that date, the lack of U.S. and British material aid will start to bite. Not much - but at this point the USSR has
nothing to spare. The British position in the Middle East will collapse, forcing the USSR to allocate forces there, and freeing Axis forces for the Eastern Front. There will be no Tunisian campaign, freeing even more forces for the East. The Soviet victory at Stalingrad was very costly and difficult. Adding substantial Axis forces would make it even more costly and less decisive - if it is a Soviet victory at all.
Eighth: By mid-1943, Britain will be exhausted. She will have lost half of her overseas possessions and supporters. Food shortages will be severe. Very probably, Germany will renew the "Blitz". It may not be especially effective, with Britain now having radar-guided nightfighters, but London will again be ravaged. Churchill's bravado of 1940 will appear increasingly hollow.
Ninth: By mid-1943, the USSR will be severely strained. The Soviets can still resist Axis attack, but that's all; the Axis will still call the tune in the East, and Soviet losses and shortages will be mounting to crisis levels. There will be no aid - and OTL, by this time, aid was substantial. It replaced much (sometimes all) Soviet production in key areas, freeing labor for other areas or military service. With the USSR essentially on its own, that means fewer shells, fewer tanks, fewer soldiers. A lot fewer - perhaps 25%.
At this point, there is somewhat of a stalemate. The Axis cannot invade Britain, and their own losses and limitations bar any decisive further blow against the USSR, though they can probably make local pushes. But there is very little the Allies can do offensively.
The next likely change is political, with Spain or Vichy France (or both) joining the Axis. There are conflicts between them, but if Spain joins, Vichy France may join to prevent its possessions being given to Spain.
OTL Vichy France actually considered declaring for the Axis in early 1942; with America not in the war and explicitly neutral, and the Axis much more successful, this decision might go the other way.
This would be a calamity for Britain - it would give the Axis control of the African coast from Morocco to Dakar. Gibraltar would be neutralized. Axis navies, augmented by much of the French navy, would disrupt British control of the Atlantic.
Portugal might also join the Axis, again from fear of Spanish ambitions, and also due to the quasi-fascist politics of its regime.
This would leave Britain practically under siege, cut off from all outside support except Canada, with nothing to show for three years of war but nearly continual defeats.
I think at this point either Britain or the USSR or both would try to make peace. Hitler would accept a status quo peace with Britain; with the USSR he might demand some further withdrawals - say from Leningrad and Karelia, plus future tribute of oil, grain, and minerals. Mussolini would want East Africa back, if the Italians haven't recovered it themselves. (I don't think Hitler would bother helping them.) They might manage it if the British forces in the theater collapse as suggested above.
Probably the war ends in mid-1944 with an Axis victory on the lines above, and Japan still bogged down in China.