Social democracy's practice has always been the administration of capitalism. The Second International's collapse in 1914 proved this materially: when imperialist war arrived, they chose nation over class. Democratic socialists on the other hand may hold socialist aims in theory, however utopian. Social democrats do not. Their program accepts capitalist property relations as permanent. Their project is the rationalization of exploitation, not its abolition: distributing a portion of imperial superprofits to the labor aristocracy of the core to stabilize the system. This is not reformist socialism but capitalist management.

Isn't everything/anything existing under capitalism financed by imperial rent? How is it different to China, for example?

The mechanism matters. The Nordic welfare state (the alleged shining example of modern social democracy) is financed by extractive capital operating in the periphery. Swedish and Norwegian firms control mines across Africa that extract cobalt, copper, and rare earths under conditions replicating colonial relations. This creates a material basis for class collaboration at home. China's path is the inverse. China was subjected to imperial plunder for a century. The revolution under Chairman Mao broke that dependency and built an independent industrial base. Today China offers an alternative development path to the periphery through initiatives like the Belt and Road, free of IMF conditionalities and enforced dependency. That is anti-imperialist practice, not imperial rent extraction.

When has this happened? Do you have a specific example of a social democratic party turning fascist?

The argument is not that social democrats literally become out and out fascists. It is that their function in crisis serves fascism. The mask depends on surplus flow. Look at Labour in the UK today. Under Starmer, they are indistinguishable from the Tories they replaced. As imperial rent shrinks, austerity and class defense move to the forefront. They back arms deals, enforce anti-union laws, and cut public services. Across Europe, social democratic parties capitulate as the far right rises. In Germany, the SPD presides over rearmament and welfare cuts. In France, the PS collapses while Macron's center holds. In Sweden, social democrats adopt anti-immigrant positions to chase right-wing voters. Social Democracys niceties are financed by imperial plunder. When that flow shrinks, it defaults to open class defense. It is not identical to fascism, but it is the bridge: austerity dismantles the welfare compromise, creating the social desperation fascism exploits.

They didn't just preserve it, they were essential in building it... it is at least somewhat explainable given the uncertainty of the situation.

"Explainable". The SPD's choice to build a liberal republic rather than smash the bourgeois state was a class choice, not a historical accident. They did not face a binary of "chaos or Weimar." They faced a choice: side with the proletarian masses who had just toppled the Kaiser, or side with the generals, judges, and bureaucrats who served capital. They chose the latter. The Ebert-Groener pact was alignment. Using the Freikorps against the KPD while negotiating with monarchists was not a tragic error. It was clear cut counterrevolution.

This is very critical and one of the biggest issues. But again, this was a mutual thing. The KPD also refused to form any kind of front against the nazis until it was too late.

The refusal was not symmetric. The SPD held state power. They controlled the police, the courts, the army. They used that power to repress communist organizing while tolerating fascist mobilization. The KPD, by contrast, had no state apparatus. Their sectarianism was a tactical failure. The SPD's collaboration with bourgeois forces was a strategic orientation. One error could have been corrected. The other was structural.

And this characterization is in part what made it virtually impossible to form any kind of pragmatic alliance/front against the fascists... Why would they work with the SPD against the fascists when the SPD was, in practice, fascist itself?

Stalin's characterization was not the cause of the split. It was the summation of material practice. The SPD had already shown, in 1919, 1920, and 1923, that they would use state violence against proletarian organizations before allying against them with the far right. The KPD's analysis recognized that a united front requires mutual trust. The SPD had forfeited that trust through their actions. The purpose of the characterization was clarity: you cannot build a front with a force that views your destruction as a precondition for stability. History confirmed that the SPD's priority was preserving the bourgeois order, not stopping fascism. That is why the "moderate wing of fascism" label stuck it is an accurate descriptor.

Many have no issue with critial support of regimes/groups/factions for specific and pragmatic goals

Critical support is possible when goals align against a principal enemy. We support Russia and Iran against imperialist aggression not because they are socialist, but because their resistance weakens the imperial core. No such alignment exists with social democrats. Their entire project is to blunt the teeth of capital at home in order to suck dry any revolutionary potential of the proletariat. They bribe sections of the working class with concessions financed by imperial plunder to enforce a false sense of class consciousness. They are enemies through and through, just like the neoliberals and the fascists. Recognizing this is not sectarianism. It is clarity. What shoot any movement in the foot is not recognizing the enemy.

Social democracy is against revolution and pro reform. If that makes it fascist, literally everything and everyone except for revolutionary socialists are/were fascists.

The definition is not "against revolution equals fascist." The definition is material: which class interest does a force serve in the decisive moment? Social democracy serves capital. When the system is stable, it administers concessions. When the system is threatened, it defends property by force. That function is what Stalin termed the moderate wing of fascism. This worldview does not turn everyone into an enemy. It identifies the enemy correctly. The West's low revolutionary potential today is precisely the result of social democracy's historical success in channeling proletarian energy into parliamentary dead ends. Abandoning class analysis to chase broader alliances does not build power. It dissolves it.