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The events from 1933 to 1945 should have been fought in 1928 at the latest. Later it was too late. We must not wait until the struggle for freedom is called treason. We must not wait until the snowball has turned into an avalanche, we must crush the rolling snowball. Nobody can stop the avalanche! It only comes to rest when it has buried everything underneath it.
That is the lesson, that is the conclusion of what happened to us in 1933, that is the conclusion we must draw from our experiences, and it is the conclusion of my speech. Impending dictatorships can only be fought before they have taken power. It is a matter of an appointment calendar, not heroism.

[Die Ereignisse von 1933 bis 1945 hätten spätestens 1928 bekämpft werden müssen. Später war es zu spät. Man darf nicht warten, bis der Freiheitskampf Landesverrat genannt wird. Man darf nicht warten, bis aus dem Schneeball eine Lawine geworden ist. Man muß den rollenden Schneeball zertreten. Die Lawine hält keiner mehr auf. Sie ruht erst, wenn sie alles unter sich begraben hat.
Das ist die Lehre, das ist das Fazit dessen, was uns 1933 widerfuhr. Das ist der Schluß, den wir aus unseren Erfahrungen ziehen müssen, und es ist der Schluß meiner Rede. Drohende Diktaturen lassen sich nur bekämpfen, ehe sie die Macht übernommen haben. Es ist eine Angelegenheit des Terminkalenders, nicht des Heroismus.]

Erich Kästner (1899-1974) German writer, poet, screenwriter, satirist

Speech (1958-05-10), “Über das verbrennen von büchern [On the Burning of Books],” Hamburg PEN Conference
    (Source)

Collected in Kastner, Gesammelte Schriften für Erwachsene [Collective Writings for Adults], Book 8 "Miscellaneous Articles III" (1969).

The speech was given on the 25th anniversary of the 1933 book burnings in Berlin. 1928 was the first federal election in Germany when the Nazi Party had federal candidates. It only received 3% of the vote.

This quote is forwarded through social media frequently, in a variety of translations and edits, usually without citation.

(Source (German)). Alternate translations:

The events 1933–1945 should have been combated by 1928. Later was too late. We must not wait until the fight for freedom is called treason. We must not wait until the snowball has turned into an avalanche. You have to crush the rolling snowball. No one can stop the avalanche. It only comes to rest when it has buried everything underneath it.
That is the lesson of what happened to us in 1933. Impending dictatorships can only be fought before they have taken power.
[Source]

They should have been combated by 1928 at the latest. Later it was too late. We must not wait until the struggle for freedom is called treason. We must not wait until the snowball has turned into an avalanche. You have to crush the rolling snowball. No one can stop the avalanche. It only comes to rest when it has buried everything underneath it.
That is the lesson, that is the conclusion of what happened to us in 1933. That is the conclusion we must draw from our experiences.
[Source]

The events from 1933 to 1945 should have been battled in 1928 at the latest. Later was already too late. One must not wait until liberty is called treason. One must not wait till the snowball has become an avalanche. One must squelch the rolling snowball. The avalanche can't be stopped anymore [...]
[Source]

The events from 1933 to 1945 should have been stopped in 1928 at the latest. After that it was too late. You can't wait until the struggle for freedom is labeled as treason. You need to squash the rolling snowball. No one can stop the ensuing avalanche after that. It ends only after everything is ruined.
[...] Threatening dictatorships can only be stopped before they have taken power.
[Source]

The events of 1933 to 1945 should have been fought against by 1928 at the latest. Later it was too late. We must not wait until the fight for freedom is called treason. We must not wait until the snowball has become an avalanche. We must crush the rolling snowball. No one can stop the avalanche. It will not rest until it has buried everything beneath it.
That is the lesson, that is the conclusion of what happened to us in 1933. That is the conclusion we must draw from our experiences, and it is the end of my speech. Threatening dictatorships can only be fought against before they have taken power.
[Source]

The events from 1933 until 1945 should've been eradicated as late as 1928. Later is too late. We cannot wait until the fight for freedom is called treason. We cannot wait until the snowball has turned into an avalanche. We must trample the rolling snowball. Nobody can stop the avalanche. It won't rest until it has buried everything.
[...] Threatening dictatorships can only be fought before they have gained power.
[Source]