Background

If we want to take a closer look at Operation Barbarossa and its ultimate failure, I think it's important to firstly look at the political and ideological motives behind the invasion. Adolf Hitler intended to invade the Soviet Union from the onset of WW2; as early as 1923, he had postulated the necessity to attack the country in his book Mein Kampf.

For the Nazis this was going to be the battleground on which National Socialism either wins out or flounders. One of the tenants of that ideology was the concept of Lebensraum (living space), which entertained the idea that the Germans were a growing people without the necessary space or resources to continue growing. According to the Nazis that additional, needed space was to be found in Eastern Europe, which the Nazis designated as ancient Germanic soil. Problem was that those territories were already populated - by the local Slavic people, who became one of the fixations of the Nazis' racial hatred and were categorised by the latter as Untermenschen (subhumans). The German government's Generalplan Ost  postulated the genocide, ethnic cleansing and enslavement of the Slavs as part of large scale race war between the "superior Aryan-Nordic race" and the "inferior Slavic race".

These racial motives help us understand Hitler's obsession with the USSR, as well as the brutality of German warfare in Eastern Europe. It can also be safely assumed that Soviet leadership could not anticipate how strongly his racist views would influence Hitler's decision-making as Stalin still thought the war to be avoidable when it was already an inevitability for Adolf Hitler.

Now that we have that clear, let's take a closer look at the war preparations both sides undertook.

Preparations

The Germans mustered an invasion force of one independent regiment, one separate motorised brigade and 153 divisions containing a total of around three million men. They were equipped with roughly 3,500 tanks, 3,000 aircrafts, 7,000 artillery pieces, 17,000 mortars, 600,000 motor vehicles and 650,000 horses. They were strengthened by additional forces from their allies Romania, Finland, Hungary, Slovakia and Italy accounting for another 800,000 men. At the time, this was the largest invasion force in the history of human warfare. Operational command lied with the Oberkommando des Heeres (German Army High Command) under Field Marshal Walther von Brauchitsch. The entire Axis forces, 3.8 million personnel, were divided into three separate army groups: Army Group North, which would be heading for Leningrad; Army Group Centre, which would be heading for Moscow; and Army Group South, which would be heading for Kiev.

The German forces in the rear (mostly Waffen-SS and Einsatzgruppen units) were to operate under the authority of the Reich Security Main Office (RSHA), but still depended on Wehrmacht supply lines for food and gasoline. They were tasked with eliminating partisan activities, as well as with commencing cleansings of Soviet political commissars, Slavs and Jews.

Adolf Hitler and his generals intented to employ the Blitzkrieg tactics against the Soviets, which had previously proven so effective in overrunning France and the Balkans, estimating that the war could be won in a matter of 8 to 10 weeks. This estimate was based not only on their belief in the superiority of the German Armed Forces, but their belief in the superiority of National Socialism as well. Adolf Hitler considered Communist society to be fundamentally weak; there's a very famous quote of him in relation to Operation Barbarossa saying "We have only to kick in the front door and the whole rotten edifice will come tumbling down."

The German invasion plans were not only based on a quick, decisive victory, they actually depended on it. Because the Germans knew they had neither the reserves nor the resources to afford a long war of attrition with the Soviet Union, so they needed their Blitzkrieg to be successful.

As for the Soviet preparations, they were a lot less decisive than their counterparts'. Although Stalin began receiving intelligence reports warning of German invasion plans from the British, the Americans and his own intelligence services as early as 1940, he chose to ignore them; either out of distrust of the British or because he hoped that war could still be avoided if only Moscow treaded cautiously enough. Fear of provoking Berlin into launching a preemptive strike was also what kept him from significantly reinforcing his Western defence lines even when the Nazis began amassing troops at the border or conducting reconnaissance flights into Soviet territory despite the urgent pleas of two of his closest military advisors - Chief of the Red Army's General Staff Georgy Zhukov and People's Commissar of Defence Semyon Timoshenko - to do exactly that.

Stalin's general wariness can certainly be attributed to the state of the Red Army in 1941. During the 1930s Great Purge, the military had been one of the dictator's main targets. Approximately, 60% of non-commissioned officers and 80% of commissioned officers had either been suspended, arrested or killed; their replacements were often appointed for political loyalty and lacked military competence. Although Moscow began with quickly reinstating formerly suspended officers in 1940, the Red Army's command structure remained in many parts dysfunctional.

Despite those many deficits, the Red Army outnumbered the German tanks and aircrafts with an available force twice or three times their size. However, many tank units were not well equipped and lacked consistent supplies of ammunition, fuel and personnel; furthermore, the Soviet airforce was considered largely obsolete. So Moscow's numerical advantage in heavy equipment was thoroughly offset by the superior training and organisation of the Wehrmacht.