Outline:
POD: 1912: Roosevelt defeats Taft in the Republican Nomination. He also defeats Wilson by a narrow margin. When the First World War starts, Roosevelt takes the US into it. This creates a perfect storm of radicalisation and anti-government sympathies. The Democrats win in 1916, but take the country along an increasingly authoritarian path, attempting to crush socialism, trade unionism and what remained of black civil rights. The result was a socialist uprising, and a civil war, which finally ended with the end of the United States and the establishment of a socialist state, albeit one with rather more syndicalist tendencies than the OTL USSR. The somewhat decentralised nature of the revolution meant that the newborn Federation of American Workers' Republics was necessarily multi-party from the outset, and there was no 'vanguard party' to take charge of revolution meant that the new state avoided a slide into dictatorship and tyranny. The Philippines and Hawaii persisted under an increasingly brutal American exile dictatorship. (I know plausibility is pretty low, but I'm going to roll with this).
Lenin's train derailed on the way to Switzerland, and while he didn't die, he was hospitalised and ended up in Germany by the time things got hot in Russia. The Bolsheviks never won round people like Trotsky, and the Russian Revolution never entered a Communist phase. The short-lived democracy still entered a civil war, as the soviets and the Provisional Government tried to establish themselves as the authority. With a split opposition, the Whites under first Kolchak and then Wrangel, conquered Russia and established a faintly theocratic dictatorship. The removal of the isolated Tsars, and Wrangel's quiet exile of the Imperial Family to their cousins in Britain, meant that the dictatorship began to make proper strides toward industrialising the country and rebuilding the nation's reputation abroad.
Germany still lost WWI, as there was no Brest-Litovsk and Wrangel's commitment to fighting the war won him support from the other members of the Entente. While Germany collapsed and civil strife tore at Eastern Europe, the crippled Lenin tried to stir the people of Central Europe into revolution. He enjoyed his greatest success in the former Austria-Hungary. It all came to naught thanks to Russian arms and a series of stunted conservative republics were established in the wake of the ancient Hapsburg Empire. Germany became a republic and the Entente breathed a sigh of relief as socialist revolution was avoided.
As the 1920s progressed, America spread the revolution to it's old sphere of influence, overthrowing the old American corporations like United Fruit, and establishing unionised socialist republics in their wake. The old Entente backed up the Brazilian Integralists, and blocked off the spread of Yankee Socialism to the Southern Cone, but couldn't save the remnant of the old USA as the Red Navy reconquered Hawaii and the Philippines didn't need too much poking to bring them into the fold. Meanwhile in Germany, a new political party led by Theodor Duesterburg emerged which railed against 'Russian imposed republicanism' and called for the reunification of Austria to Germany and the re-establishment of historical Germanic hegemony in the Balkans and Eastern Europe. Wrangel's death at the end of the decade handed over to more fundamentalist heirs who were less able to combat this new message that was gathering steam in Germany.
That all came to an end with the London Crash. The Second American Revolution had ended New York's position as a global financial centre and London had picked up the slack. The British economy had not faired well in the aftermath of the war, but a boom in financial speculation had transformed the country, almost as totally as the Industrial Revolution had done in the last century. Credit had led to a boom, but now came the bust. Unemployment sky-rocketed and all the countries tied to London's apron strings, which was most of them, went with it. The failure of the old parties led to a boom for otherwise fringe radicals. By 1932, Duesterburg was Reichskanzler, and socialism was booming in the Western democracies. There was a coup in Russia by military figures who instituted a radical programme of public works and labour battalions which put the country on the road to economic recovery. Over the next decade, Duesterburg remilitarised Germany, annexed Hitlerite Austria, restored the Hapsburg monarchy, and began interfering in the Balkans to achieve his goals of a reborn Holy Roman Empire. Civil War engulfed Italy, Spain became a German style monarchy.
The Second World War broke out as Germany interfered in Italy's civil war, seeking to restore the HRE by bringing in Italy. Duesterburg had already raised the hackles of London, Paris and Petrograd by re-establishing the Kingdom of Hungary and helping it annex Slovakia (with Germany handily annexing the Czech portion). The latest interference in Italy was too much. Britain and France declared war on Germany, while Spain swung in behind their allies in Berlin. Russia remained cautiously on the sidelines, unconvinced by either side in the war. To everyone's surprise, Germany smashed the French armies and Paris fell swiftly. Belgium and the Netherlands fell in weeks. Spain annexed some border regions of France, annexed Portugal and got stuck in to the charnel house of Italy.
In Asia, the rise of Red America had given Japan an excuse to invade China to defeat the Communists there, and when the Germans and Spanish declared war on Britain and France, Japan signed on to take control of European colonies in East Asia. Until 1941, everything was going swimmingly for the Dreikronebund of Germany, Spain and Japan. Italy had been brought to heel, the Balkans had been conquered or given to German clients, and Britain carried on a pointless resistance in their island. Then Germany launched it's invasion of Russia and only months later, Japan attacked Hawaii. The Russians and Americans had been dragged into the war.
It was a bloody, painful slog. German forces almost reached Moscow and Petrograd before winter came and the Russian counter attack came at a tremendous cost in blood and treasure. The American factories, safe from attack in their continent, produced huge quantities of armaments at a breakneck pace and distributed them to their Allies. A new front was opened up in France in 1944 and from that point onwards it was defeat after defeat for the Dreikronebund. And in 1945, after the surrender of both Germany and Italy, the Americans dropped their secret super weapon, the Atomic Bomb on Japan and brought an end to the war in the Pacific. The war was over and the world was reshaped.
Europe had been divided in two by the war, with Germany itself being literally broken up into occupation zones by the Allied Powers of America, Russia, Britain and France. But more importantly, where the American troops had gone, they sowed the seeds of socialism. Britain's Labour government which had won office in 1931 was rewarded with a larger majority in 1945, and secured an alliance with the Americans. France's politics were reshaped by occupation and the Resistance and the Communists played a big part in building the post-war state, establishing a socialist republic. Spain, Portugal and Italy all established socialist governments. The German Democratic Republic was born out of the British, French and American occupation zones, while the German State arose in the Russian occupied East. Russia now occupied all of Eastern Europe except Greece and that slipped into their sphere with the civil war of the late 40s and early 50s which established a Russian style military dictatorship.
US is socialist but still relatively democratic. Russia has become a military dictatorship.
The end date would be about 1950. Times from 1912 to about 1950 are welcome. Godspeed, and I'm looking forward to this collaborative timeline.