One way of understanding why the 1930s and 1940s were so terrible is to look at the 1920s. In the aftermath of the First World War, two European governments fell to revolutionary regimes. The Tsarist, and then the Provisional governments fell to revolution from the left, Bolshevism. The liberal constitutional Italian government fell to revolution from the right, Fascism. In both cases, however, the revolutionary movements were stopped short of their radical hopes. Powerful constituencies were willing to tolerate some change, but rejected anything that harmed their own interests.
In the case of Russia, the peasantry formed the main stumbling block. They controlled the food supply, they formed the majority of the population, and they had gained possession of both their own land and that of the aristocracy. Communism threatened private property, their private property. So Lenin settled for the “New Economic Policy”: private property in land, private commerce in food, and government control of urban industry and international trade. There things stood until the arrival of Stalin.
In the case of Italy, multiple “old elites” formed the stumbling block. The aristocracy dominated the military and the bureaucracy, the monarchy remained an important focus of loyalty, and big business and big agriculture controlled the economy. They wanted the Socialo-Communist left and the unions destroyed, but they wouldn’t tolerate anything that threatened their power. So Mussolini settled for the trappings of dictatorial power for himself and jobs for his followers.
In the 1930s Stalin and Hitler exploited changed conditions to carry through real revolutions. For Stalin, it was the death of Lenin and the disputed succession that followed, coupled with the legacy of debates on the best path forward to an actually Communist Russia. This allowed him to play off factions within Bolshevism while mobilizing the intense enthusiasm of younger Communists. For Hitler, it was the immense shock of the Great Depression to the society and politics of the Weimar Republic, followed by the commanding needs of mobilization for war. In both cases, all the old barriers to sweeping change were destroyed.
These examples may have value in understanding why some authoritarian regimes survive while others fail. One theory holds that dictatorships born out of revolution endure because the revolution destroys the old institutions, eliminating both enemies and anyone who could provide an alternative; and because the revolutionary movement packs the institutions of power with fanatics committed to maintaining the new order. This theory may explain why Communist Cuba, Communist North Korea, Communist China, and Islamist Iran all remain standing many decades after their creation.
One thing not sufficiently emphasized by this analysis is the role of terror. Right to the end of their lives, Hitler and Stalin commanded police forces that had deeply penetrated the nightmares of their subject people. Fear compelled compliance.
Why then did these supreme examples perish? Hitler lost a war Germany couldn’t win. The Soviet Union’s rulers lost their nerve at a critical moment in 1989. Those lessons may have been lost on Western observers. They aren’t likely to have been lost on current dictators.
 Max Fisher, “How Iran’s Government Has Endured in the Face of Instability,” NYT, 21 June 2021.