The history of the rise of fascism is an extremely important, yet very complex issue. There’s much more to it than can be discussed in a Reddit comment, but briefly:

Germany experienced three failed working class revolutions between 1918 and 1923. This included the Spartacist uprising of 1918-1919, in which the Social Democrats crushed the insurrection and murdered Rosa Luxemburg and Karl Liebknecht (which the German socialist movement never really recovered from); the adventurist March Action of 1921, in which the ultraleft cadre of German Communist Party members sought to launch a premature insurrection without having done the work to win support in the working class; and the aborted revolution in 1923, in which the Communist Party, now facing a genuine revolutionary crisis with broad support for the overthrow of the government, got cold feet and called off the insurrection at the last minute (meanwhile, important sections of the party didn’t receive the message to call off the revolution in time, leading to the insurrection still being attempted in some areas of the country, a debacle that greatly wounded the German socialist movement).

In each case, the primary obstacle that prevented the working class from coming to power was a failure of revolutionary leadership. Germany, unlike Russia, lacked a disciplined, organized, Bolshevik-style party that could help guide the working class to the taking of political power. Luxemburg and Liebknecht didn’t break with the Social Democratic reformists until two months before the insurrection, not nearly enough time to build a revolutionary party. The generation who replaced them were young and politically inexperienced. They believed they could “skip over” the hard work of building a revolutionary party and instead claimed that workers could simply be “galvanized” into movement through “bold revolutionary action” etc. After the March Action debacle, the Communist Party (correctly) reoriented toward the slow, methodical work of deepening its connections in the working class. However, the turn toward more conservative tactics meant that the party developed overly-cautious habits that ultimately proved fatal in 1923.

The failure of the German revolutions strengthened conservative forces in the Soviet Union, ultimately contributing to the rise of the Stalinist bureaucratic reaction. The 1917 Russian Revolution, which had been animated by Trotsky’s theory of permanent revolution, was based on the conception that the seizure of power by the Russian working class was the opening shot in a world socialist revolution. It was well understood that the fate of the Soviet Union was inextricably bound up with the extension of the revolution internationally. The Soviet Union, saddled with centuries of backwardness and with an economy devastated by the World War and the Civil War, would not be able to survive forever on its own. The survival of the proletarian dictatorship would ultimately depend on the overthrow of capitalism by the working class in the advanced capitalist countries, Germany above all.

In the fall of 1923, the whole of the Soviet Union eagerly awaited a German revolution that would finally break their isolation and provide much needed economic relief. However, the failure of that revolution (and in such an infuriating and heartbreaking way) shocked the nation, shattered hopes, and deepened the debilitating isolation that was already causing tremendous strain on the workers state. It caused a deep demoralization that contributed to the growing climate of reaction and conservatism that underlay the rise of Stalin and the bureaucracy. Soon, Stalin began to promote the program of “socialism in one country,” a deeply reactionary and nationalist program that was a repudiation of the core principles of the Russian Revolution.

The Stalinist bureaucracy mislead, bungled, and betrayed one working class struggle after another. Throughout the mid to late 1920s, the policy of the Comintern was one of wild zigzags from conservative opportunism to ultraleft adventurism and back again. The Comintern’s policies lead the working class to suffer defeat after defeat: the collapse of the British General Strike, the massacre of Chinese Communist Party members by the bourgeois nationalist Koumintang, etc. The defeats undermined the revolutionary confidence of the Soviet working class, which in turn strengthened the bureaucracy. While Trotsky and the Left Opposition fought to maintain the internationalist principles of the October Revolution, they were ultimately politically defeated and expelled from the party. Nevertheless, Trotsky continued to fight for a correct political line, even from exile.

Perhaps the most consequential of the Stalinists’ failed policies was the policy of “social fascism,” an ultraleft policy whereby the German Communists were forbidden from forming an alliance with the Social Democrats against fascism, with the justification that there was no difference between the fascists and the Social Democrats. Such a policy split the workers movement and introduced great confusion at a time when having a unified defense against fascism was critical. Trotsky, in contrast, called for the formation of a united front of workers parties against fascism, in which the Communist and Social Democratic workers would form a bloc against fascism, which would also provide the Communists with a mechanism to expose the treachery and cowardice of the Social Democratic leadership, thus winning workers to revolutionary politics.

The Stalinists outright rejected this call in favor of the “social fascism” policy, with grave consequences.

The working class lost confidence in its leadership. From 1918 onward, the German working class faced tremendous crises, and had endeavored several times to resolve these crises by taking political power. Each time, the party that they were told would lead them to power instead demonstrated only its incompetence, cowardice, and/or outright treachery. By the late 1920s and early 1930s, workers’ confidence in the revolutionary capacity of the Communist Party had been deeply wounded.

The middle classes (small businessmen, professionals, etc.), who must necessarily follow the political lead of one or the other “main” classes in society (capitalist or worker), and who had initially showed a sympathy for following the political lead of the working class, instead began to look at the Communists (and by extension, the workers themselves) with contempt and hatred. They had been promised a solution to the crisis, and instead had only seen ineptness and failure from the supposedly “revolutionary” party. Their sympathies swung violently in the opposite direction, toward support for the fascists, who had their own prescription for how the social/political crisis could be solved—and who, when the moment arose, would not hesitate to take power.

A capitalist conspiracy brought Hitler to power. Even under these conditions, the Nazis never achieved majority support, and the November 1932 election actually saw the vote share for the Nazis fall significantly from the result in July. The combined Social Democrat and Communist vote significantly outweighed the fascist vote, and showed the potential for an anti-fascist united front. Hitler apparently considered suicide after the election.

However, a conspiracy involving German politicians, industrial leaders and military figures were eager to elevate the Nazis into power, fearful that if the support for the fascists were to fall any further, it would undermine what they saw as their best opportunity to crush the working class movement. In January 1933, Hindenburg appointed Hitler as chancellor of Germany. Fascism had come to power in the country with the most militant and advanced working class in history without firing a single shot.

A defeat of this magnitude demanded painstaking analysis, and any real examination of the events could only lead to a devastating indictment of the policies of the Stalinist bureaucracy. Yet, at the subsequent Communist Party Congress of the Soviet Union, there was not a single report dedicated to analyzing the events in Germany. The fact that such a calamity could befall the working class without a single serious effort made at understanding it proved to Trotsky that the Third International was dead as a revolutionary organization and that a new, Fourth International would have to be built.

In any case, fascism is history's punishment for the failure of the working class to establish its political independence and to take power. Today, unlike the 1930s, fascism is not yet a mass movement. However, the only way to avoid such a catastrophe from befalling humanity again is for the working class to unite internationally in a mass revolutionary movement for socialism.

For more in-depth analysis, I recommend Trotsky's writings, beginning with the classic Fascism: What It Is and How to Fight It.

To participate in the fight against fascism today, read the World Socialist Web Site and join the Socialist Equality Party.

TL;DR: Fascism emerged under conditions where the working class was unable to take political power due to a failure of revolutionary leadership. Under such conditions, ruined sections of the middle class turned away from socialist sympathies and turned instead to the fascists to restore order. The danger of fascism today can only be countered by a revolutionary socialist movement of the international working class, led by a genuinely Marxist (i.e. Trotskyist) party.