Why did Germany chose to attack USSR even when they had the non-agression pact working between them? Can we call it a betrayal?
To call it a betrayal would imply/suggest that the two countries were working together in the first place, or went into an agreement both wishing to keep up said non-aggression. The nature of German-Soviet relations in the Interwar Years were tense to say the least, the Nazi's made it clear that Communism and Bolshevism were the biggest enemies of Fascism/Nazism, and the Soviets as a result feared the German ideological animosity to them, as well as their re-armament.
The Nazi's, as the years went on, made their opinions on Communism, the Slavs, and the East more and more clear. Many of Hitlers speeches featured anti-communist, anti-east, and/or anti-soviet elements. For six years prior to 1939, his speeches would usually feature his signature message that Bolshevism is evil. Policies such as Lebensraum were pretty clear examples of German desire to strip the lands of the East from the Soviets and populate them with German Aryans. It was one of the things that Hitler always made very clear, he intended to preserve, and then enlarge, the Volksmasse (the racial stock, meaning pure Aryans) by the means of gaining more space. He made this abundantly clear on November 5th, 1937 at a meeting of the Reich Chancellery, where he made such comments, and made it clear that he was gearing up for war as early as 1937 (though likely even earlier.) In the meeting Hitler made sure to stress the need for urgency in rearmament and the imminence of armed conflict in East-Central Europe. This is largely just an example of the German foreign policy, and it should therefore be no secret that (Nazi) Germany never really intended to be peaceful and cooperative with the Soviets. Hitler (and Goering, who, in 1936, said 'the showdown with Russia is inevitable' when he was trying to speed up rearmament.) have been planning an eventual conflict with the Soviets since, well, the Nazi's came to power.
Likewise, in the German public, everyone right (on the politic spectrum) of the communists seemed to believe in the backwardness of Slavs in general. Hatred of Communism was accepted, which made the distaste of the USSR go hand-in-hand, and Nazi Propaganda promoted the view that Communism and the Soviet Union were the enemies of National Socialism and of the German people as a whole. There was nothing mistaking the Soviets that there was a clear animosity emanating towards them out of Germany.
As 1939 loomed over, in the months after the "Rape of Czechoslovakia", Hitler began sizing up Poland. This is where the Molotov-Ribbentrop Pact began to be conceived. Hitler accepted now that after his debacle in Czechoslovakia, both Britain and France were very likely to move against him should he threaten and invade Poland. However, he was weary of moving against Poland due to the strategically long border that Poland shared with the Soviet Union. By May, 1939, Hitler had begun realizing the importance of securing the benevolent neutrality of the Soviet Union should he wish to invade Poland. It was likely that Britain and France could convince the USSR to stop German aggression there, or for the Soviets to intervene anyways out of their own interests. By June 6th, 1939, Hitler had stopped including in his speeches his customary diatribes against the menace of world Bolshevism, and switched to berating the Western Democracies. Hitler intended to start and move towards possibly opening some means for a possible (I say possible because Hitler even now still had his reservations) deal with the USSR that could avoid him a two front war and the guarantee of a clean invasion of Poland. In short, he wanted to avoid a Two-Front war.
And Stalin did as well. Over the previous few years, he had carried out violent purges of his top generals, factory managers, and army officers. The Soviet Union was most definitely starved of any real Generals or Officers who had any kind of direct warfare experience, rendering the country's preparedness for war lamentable. Since Stalin knew by June, 1939 that Hitler intended to invade Poland by August or September, he wanted to ensure that it went no further than Poland. Yet he was no fool, he was aware that Hitler could not be trusted and he knew that German ambitions likely were not limited to just as East as Poland. He wanted to buy time so the country could regroup and rebuild, re-equip the Red Army, fix the arms production and train better officers in anticipation for the German invasion he feared would eventually come. Stalin had two options: Try and ally with the Western Allies for support to stop Hitler, or sign a non-aggression pact with Germany to buy time to prepare for war. But since the Western Powers were wary and hesitant, wanting to avoid war and considering Stalin as unreliable, Stalin was won over by the eager Ribbentrop and German foreign Office.
So, on March 10th, 1939, Stalin declared that he would not be willing to come to the rescue of the Western capitalist Powers if they got into a conflict with Germany, accusing their policy of appeasement of strengthening Germany's long-term goal of attacking the USSR. On May 3rd, 1939, Stalin dismissed Maxim Litvinov, his long-time Foreign Minister (who was in favor of collective security and positive relations with the West), and replaced him with his hard-line henchman, Vyacheslav Molotov. Stalin was sending very clear messages to Germany that he wanted some sort of compromise with them over the looming issue of Poland.
Eventually, the hints from Moscow grew ever stronger, and Ribbentrop, the German foreign minister, began negotiating trade relations with the Soviets. He and Molotov eventually agreed that such economic agreements should have a political dimension, so by early August, 1939, Ribbentrop and Weizsacker drew up a plan to partition Poland with the Soviets. Stalin agreed to the terms on August 21st, invited Ribbentrop to Moscow, who arrived on August 23rd, and signed it that same day. Ever since it, was known as the Ribbentrop-Molotov Pact, the non-aggression pact between Germany and the Soviet Union.
But again, one cannot mistake this pact as the two powers forming an alliance. These are two countries that just spent the last 6 years mutually vilifying each other in public, and had been major backers of the two opposing sides in the Spanish Civil War. It was a huge shock to not just the West, but also the world, that two countries that hated each other so much, had just agreed not to fight each other. In reality, the pact served the interests of both Hitler and Stalin, and was more of considered (at least, likely by Stalin) as a 10 year intermission before a possible war between them. From Hitlers point of view, securing Soviet acquiescence was vital to preventing a two-front war, and from Stalins point of view, it gave the USSR respite and a chance to rebuild, as well as the juicy possibility that the capitalist powers of Britain, France, and Germany might fight a war of destruction among each other.
As Poland, Norway, Denmark, Belgium, the Netherlands, France, and the Balkan states all fell in the wake of the German army throughout 1939-1941, Germany began to turn its eye on the Soviet Union. The failure to break Britain in the Battle of Britain left Germany with its last enemy to strike down, the USSR.
Now, the Germans invading the USSR shouldn't be a huge surprise given how clear the Nazi's made their hatred of Bolshevism be. What should be considered is the timing. Hitler feared that every year and month that the Soviet Union was left untouched, the stronger it will grow. The summer of 1941 was likely one of the earliest dates that the Germans could strike at the Soviets, thinking that every year they wait the Soviets would strengthen. Given the pitiful (yet ultimately successful) performance of the Soviet army in Finland (known as the Winter War), and the weakness of the USSR's officers and military, Hitler thought that it was the prime time to attack. He had crushed France, and secured his southern and northern flanks. It was likely one of the best chances to attack while Britain was still down and bleeding. He thought that the Soviet Union just needed "One good kick, and the whole rotting structure will come tumbling down." So, on June 22, 1941, the Germans launched Operation Barbarossa, their invasion of the Soviet Union.
To summarize, the Germans eventually broke the non-aggression treaty and invaded the Soviet Union because it was what the Germans were largely planning to do in the first place. The Germans made it clear that the Soviet Union, Communism, and especially 'Bolshevism', were the real enemies of Fascism and Nazism. Hitler spent years before 1939 making speeches about the inferiority of Slavs and communism, making out the USSR to be clearly his ultimate enemy. The Nazi ideological policy of Lebensraum demanded that the Soviet Union be defeated, conquered, and settled by Germans to provide Aryan Germans with "Living space". Therefore, neither Hitler, or Stalin, were fooled by the Molotov-Ribbentrop Pact, and knew a war between them would arise at one point or another. It was more of a temporary ceasefire than a real lasting non-aggression pact, the pact itself was the surprise, the German breach of it, was not.
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