The
Islamic rebellion of 1937, in Xinjiang, happens slightly earlier (before any full-scale war breaks between China and Japan), and leads to a short, local, war, between USSR and Republic of China. The Chinese are forced to relinquish Xinjiang. But, there is a massive international outcry. Soviets leave LoN, while the USA, pressed (among others) by their Chinese and Japanse communities, put sanctions against Moscow.
In Japan, at the time, some in the Army wanted to focus on the Soviet threat rather than China or the Pacific, and/or expand in Eastern Siberia. While Chiang was first and foremest anticommunist, and feared both the CPC and the Soviets. Under German auspices, China and Japan concluded a non-agression pact, which let the Japanese occupy Manchuria to "help to defend" the Chinese sovereignty there.
Few months after, in France, La Cagoule manages to assassinate Marx Dormoy, the Minister of the Interior, who mostly brought it down, by planting a bomb. Unfortunately, children are killed or mutilated by the bomb, causing a national outcry. The NKVD seizes the opportunity, by giving anonymously proof that Italy and Germany were giving weapons to the Cagoule, to attack Spanish Republicans in France, their helpers, German and Italian opponents, and French left personalities. That event, combined to the outcry about Guernica bombing, allows the French Communists (who are in the government) to convince Blum to intervene directly in Spain in 1938.
Still, when Germany wants to take the Sudetes in Chechzslovakia, France and GB accept the Munich "compromise" because the British are really unwilling to go to war, and in France, the Front Populaire broke up, isolating the Communists. French are only in a proxy war against Germans and Italians in Spain, along the Soviets.
In 1939, the Germans convince the Poles to renew the non-agression pact between them, in exchange for annexing Dantzig and building an extraterritorial motorway between it and the German mainland. ATL, the Poles fear the Soviet agression more, because of Xinjiang, and see the French as Soviet-aligned and the British as cowards, so they accept, in fact, German protection.
In Spain, against French army, the Nationalists have to capitulate, and go in exile.
Then, the Battle of Khalkin-Ghol happens between Japanese and Chinese on one side, and Soviets on the other, both sides accusing each other of agression. Under the guise of "retaliation", some IJA officers take upon themselves to attack Soviet territory, pushing the Soviets to declare war on both China and Japan. It's unclear who really attacked first, but a good portion of the American opinion favored Japanese and Chinese.
Red Army, already mobilized, overruns Manchuria. Some atrocities, and the mistreatment of American civilians, push the USA to step up sanctions against the USSR and start lend-lease to Japanese and Chinese.
Four months later, in Europe, Germany (seconded by Poland), thinking the Soviets are too occupied in Asia and the Westerners won't react, presents its ultimatum to Lithuania, asking for Memel, while the Poles ask for the recognition of their possession of Vilnius.
France, emboldened by its success in Spain (which proved her she doesn't need British support to act), and better prepared than OTL (they started to expand the Maginot Line to North, even though it's not yet finished), threatens Germany to declare war if she invades Lithuania. The Soviets, hoping to cause a war between Germany and France, discretly tell the Lithuanians they will, too, support them (they know that the current French government, of Daladier and Reynaud, won't want to appear as Soviet allies, so they act secretly). Germans don't take the French threat too seriously, they are wrong.
When Lithuania refuses the ultimatum, and is invaded by Poland and Germany,
the French declare war to Germany. To the surprise of everyone else, the Soviets, who had a secret defence pact with Lithuania, follow suit, and invade Poland. Italy, seeing France alone, decides to take its chance and declare war on France and USSR. Japan and China declare war on France, in the hope of invading Indochina (which will be rather easy for them).
The Germans, panicking because of the two-front surprise war, decide to take the (short) window of opportunity to strike at France through North, before the Maginot Line is complete, and invade Belgium and Luxembourg for that (not Netherlands). Like in WWI, that act convinces the British to declare war on Germany and Italy. The US put sanctions against Germany for the invasion of Belgium.
The events of 1939 drive a wedge between Americans, and British and French. Even if the US condemned the German actions, they still support Japan and China against the USSR, which they see as the main threat, and suspect France of being too much Soviet-friendly. On top of that, China, in 1940, decided to revoke all inequal treaties and concessions, but signed a trade convention with the US allowing them to keep their shares in Shanghai concession. In exchange, the US supported their decision to revoke concessions.
In 1940, the Germans mostly fight a defensive war against Western Allies and Soviets, respectively in Belgium and at the Polish border. They support the Antonescu putsch in Romania to get more oil, prompting Soviets to declare a "war of liberation" of Romania, while Hungary joins Germany (and grabs Transylvania). Romanians, fearing Soviets more than Germans, mobilize against the USSR. Bulgaria, in exchange for Dobrudja, enters war along Romanians, Hungarians and Germans.
The Western Allies, hoping to cut off Germany from its supplies of iron ore, decide to take control of Narvik and the Swedish mines by surprise. The Germans had a plan to do the same in Norway, but British and French strike first. That operation is a fiasco, pushing Norway, Sweden and Denmark (reluctantly) in the Axis and prompting the USA to apply the same sanctions against London and Paris, it applied on Berlin after the attack of Belgium (because basically, Western Allies did to Norway what Germans did to Belgium), worsening relations between Washington, Paris and London.
The Americans also increase the land-lease to China and Japan, and send the Flying Tigers help China (their true reason is to keep some influence in China, where Germans and Japanese are in a better position). The Germans, who were already ready to attack, intervene to "help" the Scandinavians, and expel the British-French force in months.
The Soviets, fearing now an Axis invasion through Finland, order Finland to cede them Petsamo and the Eastern Karelia, but the Finns refuse, so Soviets attack. The Winter War between all Scandinavians (with German support) and Soviets is a disaster for Soviets. Who made themselves even more unpopular (especially in the US) by committing more atrocities, both in China, and in Poland, where they have to deal with the Armia Krajowa (supported by Nazis and Polish-American sympathizers).
In 1940-1941, the tensions worsen. Soviets, British and French wage a naval war against German and Japanese shipping, and the Axis responds in kind. Irish and American shipping are very often targeted by the Allies, and each time, British and French offer apologies to American government each time, but continue the attacks nonetheless. Soviets don't apologize at all. Though,
Germans target American shipping to Britain as well.
In 1941, something starts a war between the Soviet Union (and possibly France and/or Britain as well) and USA.
