Fascism: What It Is and How to Fight It, written by Leon Trotsky in 1933, is a critical analysis of the rise of fascist movements in Europe, particularly focusing on Nazi Germany under Adolf Hitler. Trotsky argues that fascism is not a sudden, irrational outburst of violence but rather the result of deep structural crises in capitalist society, which the ruling class seeks to resolve through authoritarian means. He explains that fascism arises when the traditional political establishment is unable to suppress revolutionary movements from below (such as workers' uprisings or socialist revolutions) and when the capitalist system is threatened by economic collapse.

Trotsky highlights that fascism is a tool of the bourgeoisie, aimed at preserving the existing social order through terror, militarism, and the suppression of democratic freedoms. However, it is not simply a reactionary force; it has its own set of internal contradictions, as fascists simultaneously defend capitalist interests while attacking the political establishment and the working class.

Trotsky warns of the dangers of fascism's rise and stresses the importance of building a united front of the working class and the Left to prevent its spread. He critiques both Stalinism and social democracy for their failures to adequately respond to the fascist threat, emphasizing the need for a revolutionary approach to combat the fascist menace.