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At the beginning of WW2, Nazi Germany orchestrated the infamous Gleiwitz Incident, which was used by Hitler to publicly declare, that since 5:45 we are returning fire.

The invasion of Poland triggered the UK and France to declare war on Germany. The USSR, on the other hand, was in a non-aggression pact with Nazi Germany.

Was there any justification provided to the public by the government or Hitler himself as to why the Wehrmacht was invading the USSR? Or by this stage of WW2, had the war already progressed beyond the point where there was any need for reasons as to why attack another country?

Both the German and the English Wikipedia are quite silent on the topic of the public justification of Operation Barbarossa. They discuss the planning and the timeline, yet there is no mention of any propaganda activities.

Only Hitler's views on Bolshevism, which were made public in his book, are discussed.

Hitler gave his reasons to the German people via a radio broadcast on the morning of June 22nd, 1941.

At 0500 GMT, an hour after the invasion began, the Nazi Minister for
Propaganda, Joseph Goebbels, went on national radio to read a
proclamation by Adolf Hitler

The proclamation can be seen here in full. Basically, Hitler argued that the Soviets were a threat to Germany and had broken the peace. Thus, Germany was 'forced' into a preemptive strike.

Below are some excerpts for those who (understandably) don't wish to wade through the entire proclamation:

Moscow not only broke our treaty of friendship, but betrayed it!

I was forced by circumstances to keep silent in the past. Now the
moment has come when further silence would be not only a sin, but a
crime against the German people, against all Europe.

Today, about 160 Russian divisions stand at our border. There have
been steady border violations for weeks, and not only on our border,
but in the far north, and also in Rumania. Russian pilots make a habit
of ignoring the border, perhaps to show us that they already feel as
if they are in control.

During the night of 17-18 June, Russian patrols again crossed the
German border and could only be repelled after a long battle.

Now the hour has come when it is necessary to respond to his plot by
Jewish-Anglo-Saxon warmongers and the Jewish rulers of Moscow’s
Bolshevist headquarters.

There was no build-up propaganda prior to the invasion - after all, why give the Soviets any hints? On the contrary, German propaganda focused on attacking Britain.

The absence of any reference to Bolshevism, Stalin and his empire,
even for the purpose of negative integration or diversion from the
evident failure of the regime’s anti-British strategy, had been
conspicuous in the output of NS propaganda for a while – and it
remained so until 22 June 1941, that is after the start of the war in
the east....Although the time of the invasion of the
Soviet Union was drawing near, Goebbels continued to deceive not just public
opinion but everyone involved in the dissemination of information.

Source: Aristotle A. Kallis, Nazi Propaganda and the Second World War