Like IOTL, the Second World War did not begin suddenly with a single event. Rather, it represented the coming together of multiple conflicts, with each prelude war being subsumed into part of a single, world spanning conflict. In this regard, the war began with the outbreak of the Spanish Civil War, expanded with the outbreak of the Second Sino-Japanese War and Nazi expansion in Central and Eastern Europe, before the war became a conflict of great powers with the Nazi invasion of the Soviet Union.
Nevertheless, it is this event that united the various regionalized conflicts into a generalized world war. And it was a war that was truly globalized, with major fighting occurring on every continent, and at the height of the war only a handful of nations were not seriously involved fighting on one side or the other.
The events that led to the eventual outbreak of general war across the Eurasian continent can be summarized quickly. With the collapse of Czechoslovakian resistance, in spite of the heroic efforts by the Communist-led Popular Front to overturn the center-right’s capitulation to the Nazis, German aims turned towards Poland. The fragile Republic of Poland, controlled since the mid-20s by a succession of forgettable military strongmen, had been quick to find common cause with the Nazi state. The Polish government has lived in fear of Soviet invasion for two decades, and saw their German neighbor as a bulwark against that eventuality. Hence, the Polish government pressured Rumania to block Soviet attempts to supply Czechoslovakia.
Unfortunately for the Poles, Hitler found that they had outlived their usefulness with the successful annexation of Czechoslovakia. The Polish
untermenschen were in possession of valuable living space and resources that rightfully belonged to the Aryan master race, and it was not hard at all to convince the French rightists to disentangle themselves from the fate of Poland in the summer of 1938.
The difficulty now was to not bite off more than could be chewed, at least at first. Hitler was still smart enough to realize that Stalin would recognize any attack on Poland as a direct assault on the Soviet Union itself in spite of the hostility between the two governments. The trick would be to convince Stalin to stay his hand.
In the months leading up to the signing of the Molotov-Ribbentrop Non-Aggression Pact, the Nazi regime’s public rhetoric shifted towards settling accounts with Britain and the French. Though this created discomfort among his erstwhile allies in France, the Nazi foreign office was able to convince their French counterparts that it was mere rhetoric, while ensuring to their Soviet counterparts that Hitler’s public rhetoric matched the real aims of the Nazi regime.
The pact was signed unceremoniously as a mere attempt at ensuring the peace between two nations, but its secret protocols established a much more sinister agenda. The barbarian horde would be appeased: Stalin’s government accepted German hegemony over Poland and the Balkans, and they would pay tribute in the form of vital trade to strengthen both economies. Raw materials would flow west in exchange for capital goods and technical assistance.
The pact was signed in late 1938 to little fanfare. However, when American intelligence intercepted transcripts of the secret protocols, there was a crisis of confidence in Foster’s government. Uninformed of this by the Soviet government, and indeed feeling personally deceived and betrayed by Stalin, Foster nonetheless defended Soviet policy before a closed session of the Central Committee, as well as within the Politburo.
This would be Stalin’s “great gamble,” as Foster described it, to put the Nazi regime onto a collision course with the two great bourgeois imperialist powers. They would exhaust each other in futile bloodshed over a revanchist cause, sowing the seeds for a new a revolutionary wave in Europe, to be followed by a decisive revolutionary war, in which the Comintern would rally to the defense of new revolutionary uprisings.
Through great effort, he convinced the Left Opposition to not go public with information on the secret protocols, though they didn’t like it one bit. Foster found himself skating on thin political ice with all the factions of the Workers’ Communist Party. The right opposition threatened to defect to the DFLP over the concessions he made to the Left. The Browderists questioned his commitment to the Comintern’s united front. His own faction on the center-left were chafing under his micromanagement, and the Left all but denounced him for allowing America to be backdoored into a pact with the enemies of the workers’ republic.
The spring of 1939 came all too soon. And as the snow melted, so did Poland’s fortunes. Abandoned by the world community, as Fascist and Communist made common cause to seal the doom of the Polish people, they fought gallantly against the German blitzkrieg. But their fight was futile, and in a month Germany had obliterated the armies of the Polish Republic, and put the country under their yoke. What would follow under
Generalplan Ost would be nothing short of genocide.
By the end of 1939, all of Eastern Europe, from Danzig to Trieste, all the way to the Soviet frontier was under Nazi domination. Eastern Europe was filled with a Jewish population that Hitler no longer had any patience for. Persecution of Jews, Roma, Slavs (with some exceptions provided to the reliable Croats and others who Hitler felt sufficiently useful) and other undesirables intensified, both by the Germans and their puppets. The first seeds of the Final Solution were sown.
The forces the Germans marshaled from their “allies”, and the resources they obtained, were all organized to build a war machine for a singular purpose. Hitler, as gleeful as a schoolboy for pulling one over on Stalin, drew his plans against the Soviets.
The British and the French had paid their tributes to the barbarian horde as well. Trade, technical assistance, recognition of the German sphere of influence in Eastern Europe; it was all quite valuable. And in time, they might join the Anti-Comintern Pact of their own accord. But for now, they were content to bankroll the
Drang nach Osten, and that was sufficient. The time to deal with the Bolshevik menace was fast approaching, and everyone in the world seemed powerless to stop it.
Which is not to say that it went unnoticed. You cannot assemble the world’s largest army on someone else’s doorstep without someone else noticing. British and French intelligence knew it. American intelligence knew it. Japanese intelligence knew it. Even Soviet intelligence knew it, in spite of Stalin’s continued hammering of those who he felt threatened the fragile peace with Germany with their “paranoia”.
The habitual paranoiac and his inner circle seemed to be the only ones convinced that war wasn’t coming to the Soviet Union. While they had made preparations in modernizing the military, they had been less urgent than the situation called for. He was convinced, based on the German’s successful deception operations, that the units stationed on the Soviet frontier were phantom units, to deceive the French into unpreparedness for the coming invasion. The units were really preparing their positions to strike in Belgium and at Verdun. But in reality, the German battle plans against France were the real phantoms. They were in denial, and Stalin most of all wanted to believe that German bellicosity to the west would result in war.
It did not. At 3:15 A.M., on Sunday 19 May 1940, the
Luftwaffe commenced bombing strategic command targets in Byelorussia and Ukraine. Within hours, Axis forces crossed the border into the Soviet Union.
The German government announced the invasion to the world later that morning, with Goebbels delivering the radio address himself. Response in Western Europe was muted, with many media sources and the governments endorsing the German line that the invasion was a pre-emptive attack to stop imminent Soviet attack.
In the UASR, the Central Committee called an emergency session of both the CEC and the Convocation of Soviets. The American ambassador in Berlin delivered a final ultimatum to the German government: unless the German government indicated, by noon on 20 May, a willingness to unconditionally withdraw all forces from the Soviet Union, a state of war would exist between America and Germany.
The ultimatum went unheeded, and by the end of the legislative day, the Congress of People’s Deputies had unanimously passed a declaration of war against Germany. Under American direction, the governments of Haiti and Quisqueya both separately declared war that evening, even though both were in the process of petitioning to join the UASR. The governments of Mexico, Nicaragua, Panama, El Salvador, Columbia, Chile and Argentina declared war on Germany the following day.
The rest of the Anti-Comintern Pact, with the exception of Japan which maintained neutrality for the time being, joined into the war. This meant that Brazil would be at war with America’s allies in South America, and it wasn’t long before the remaining capitalist states on the continent joined in the anti-communist crusade. With the Republic of China joining the Comintern, and America pledging to an undeclared war against Japanese aggression, the war became truly global.
Most likely early 1950's Japan is an example, until it democratized... China too if a PRC analogue came up. Even if it's just a leftist ROC that is a one popular front state it already counts. In a way, partly because of Trotsky's success here in TTL of starting the integration process of the Comintern when Stalin died in the war, we can see America propping the Soviet zone indirectly at least for easing tensions. We do have the Communist Unity Party here for that. And Nixon who caused the Irish Missile Crisis in the first place is from the Uniates based on my speculation. He's a pro-Soviet guy here. 
