So let's start with the standard dystopian 30s America POD of "FDR gets assassinated, president Garner"
I imagine Garner not having the commanding persona of Roosevelt to calm the nation, and pursuing a less aggressive agenda without the insane amount of policymaking Roosevelt enacted, leading to both a somewhat less robust recovery by 1934 and also way less public confidence in the president. This causes the Democrats to take significant losses in the midterms, rather than their OTL gains (at least in the House)
These losses scare Garner and the reduced Democrats, who think the high spending and debt is what's unpopular, so they do the 1937 cutback two years early and sabotage their own recovery. No 2nd New Deal obviously
The still terrible economy leads to Huey Long trying to primary Garner, he fails to get 2/3 and then runs an independent left-wing campaign that gets the support of a lot of independent groups after Floyd Olson's campaign collapses with his death
Meanwhile, the desperate times have led to millions of people flocking to the Technocracy Movement. Howard Scott's newspaper is selling like crazy, he's published books, he's gotten very famous. Prominent enough and popular enough among left-wing circles to get various advisory positions helping for example Upton Sinclair's projects in California (governor Sinclair won in 1934 due to desperation leading people to turn to radicals) while simultaneously distancing himself from the super radicals and crating an aura of intellectualism and reason that leads a bunch of right-wingers to view him as the least dangerous of the group
So Huey Long wins the 1936 election against Garner and the broken Republicans, and proceeds to govern incredibly incompetently and flagrantly ignores the Supreme Court
Long has trouble filling out his cabinet, so he gets Howard Scott, who's become incredibly powerful and influential, to be his Secretary of Commerce or something
Corporate America, desperate for a savior, turns to general George Van Horn Moseley, who attempts to lead a fascist coup against the left-wing regime. They succeed in killing Long, his VP, and most of his cabinet, but they miss Scott who they never considered that important (low level position, kooky background) or dangerous (not a committed leftist)
The coup however is not broadly supported, and after the initial attacks, there's a mass uprising and the rest of the U.S. army refuses to back Moseley and his is killed. Scott is now president of the United States
The Communists use this as an opportunity to rise up and try to stage a revolution. President Scott easily crushes the revolution.
Now seen as having saved the Republic from the left and the right, he manages to get both the people on his side as well as the elites, afraid of further chaos, to back him. He now has the backing to purge any leftist dissenters, claiming they're communists, or right-wing dissenters, claiming they're fascists
Scott proceeds to solidify his control over the U.S. government, and uses his power to seize control of the media. He has massive jobs programs which succeed in temporarily lowering unemployment, leading to great popularity. He eventually arranges a nationwide referendum on a new constitution in 1940 which passes and formally ends democracy in the US and turning the country into the North American Technate.
Meanwhile, Germany has been taken over by a more standard right-wing military dictatorship than Hitler, someone who's less aggressive and so Europe is more of a 3 way Cold War between the British/French, Germans/Italians, and Soviets. Japan's slowly gaining ground in China, but not attacking the European empires. They still have high tensions with the Technate.
By the end of the 40s, Scott both feels secure enough in total political and economic control of America, as well as weird cultural reshaping programs, to launch his invasion of Mexico, the Caribbean, Canada, and Central America.
So basically everyone declares war on the North American Technate because they're obviously insane and planning to conquer huge swathes of territory. Facing off against the British Empire, French Empire, re-armed Germany, Imperial Japan, the Soviet Union, Italy, and basically all of Latin America, the Technate bankrupts itself to hold them off. Due to being on a shaky economic foundation to begin with, and tired of increasingly bizarre totalitarianism, a series of massive revolts erupt across the Technate.
The victorious anti-Technate powers each back some of these revolts, and soon the former U.S. is carved into various client states or other breakaways by the end of the war.