Terrorism and Communism: A Reply to Karl Kautsky (German: Terrorismus und Kommunismus: Anti-Kautsky; Russian: Терроризм и Коммунизм, Terrorizm i Kommunizm) is a book by Soviet Communist Party leader Leon Trotsky. First published in German in August 1920, the short book was written against a criticism of the Russian Revolution by prominent Marxist Karl Kautsky, who expressed his views on the errors of the Bolsheviks in two political tracts, Dictatorship of the Proletariat, published in 1918 in Vienna, followed by Terrorism and Communism, published in 1920.

Trotsky's book, the first English edition of which bore the title The Defense of Terrorism, dismisses the notion of parliamentary democracy to govern Soviet Russia and defends the use of force against opponents of the revolution by the dictatorship of the proletariat and working class masses.

Early in August 1918, mere months after the November 1917 Bolshevik Revolution which brought the Communist Party to power in Russia, European Marxist Karl Kautsky published an oppositional political tract, The Dictatorship of the Proletariat, which charged the Bolsheviks with fomenting civil war due to their failure to uphold the norms of universal suffrage. Kautsky's pamphlet, Die Diktatur des Proletariats (The Dictatorship of the Proletariat), asserted that the only way to control the growth of bureaucracy and militarism and to defend the rights of political dissidents was through parliamentary democracy based upon free elections and that V. I. Lenin and his political associates had blundered badly by departing from democratic practice in favor of a restricted electorate and the use of extra-parliamentary force.

The Bolsheviks had sought broad international support from socialists around the world with a view to the achievement of worldwide revolution on a comparatively short timetable and Kautsky's sharply critical book was regarded by Bolshevik Party leader Lenin as a rank betrayal of the Russian Revolution and a grave threat to the revolutionary socialist mission. Countering the public opposition by the world-famous Marxist Kautsky was regarded as pivotal. Lenin was quick to respond to Kautsky's book with a bitter counterattack of his own, the short book The Proletarian Revolution and the Renegade Kautsky, written in October and November 1918.

Lenin railed against Kautsky's pamphlet as the "most lucid example of that utter and ignominious bankruptcy of the Second International about which honest socialists in all countries have been talking for a long time. He charged that Kautsky had stripped Marxism of its "revolutionary living spirit" by his rejection of "revolutionary methods of struggle," thereby turning Karl Marx into "a common liberal." Quoting Frederick Engels as an authority, Lenin contended that "proletarian revolution is impossible without the forcible destruction of the state machine and the substitution for it of a new one." He minced no words in asserting as a "plain truth" that:

"Dictatorship is rule based directly upon force and unrestricted by any laws.

"The revolutionary dictatorship of the proletariat is rule won and maintained by the use of violence by the proletariat against the bourgeoisie, rule that is unrestricted by any laws."

Lenin rejected Kautsky's parliamentarian and legalistic interpretation of the ideas of Marx and Engels, contending that Kautsky knew well that the duo had "repeatedly spoke about the dictatorship of the proletariat, before and especially after the Paris Commune" of 1871, and that Kautsky had intentionally made a "monstrous distortion of Marxism" to bolster his own moderate political ideas.