History in 30

Though Germany was technically Russia's ally, Soviet leader Joseph Stalin had no delusions that they were friends. Instead, he used this time to build up his forces for what he saw as an inevitable invasion. First, on the heels of the German invasion of Poland in September 1939, Stalin had his troops invade and reclaim the land Russia had lost in World War I. Next he turned his attention to Finland, which was only 100 miles from the newly named Leningrad. He initially tried to negotiate with the Finnish government for some sort of treaty of mutual support. When this failed he simply invaded. While the giant Russian army ultimately won, the fact that little Finland held them off for three months demonstrated how poorly organized the bigger force was.
Initially, Stalin believed he had several years to build up his army before Germany would invade, figuring it would at least take the Germans that long to conquer France and Britain. However, when France fell quickly in 1940, it seemed he might have miscalculated, so he again sent Molotov to Berlin to stall for time. Meanwhile, Hitler trained his sights on Britain, turning his attention to destroying the Royal Air Force as a prerequisite for the invasion of Britain.
What Stalin did not realize was that Hitler had simply over stretched himself in Yugoslavia and only planned to delay the invasion by a few weeks. Hitler aimed to destroy Stalin's Communist regime, but he also hoped to gain access to resources in Russia, particularly oil. Throughout the first half of 1941, Germany dug in to safeguard against an Allied invasion of Western Europe as it began to mobilize millions of troops to invade the Soviet Union. Stalin even refused to believe the report of a German defector who claimed that the troops were massing on the Soviet border at that very moment.
The Soviets were so caught by surprise at the start of the attack that the Germans were able to push several hundred miles into Russia across a front that stretched dozens of miles long, reaching the major cities of Leningrad and Sevastopol in just three months. The first major Russian city in their path was Minsk, which fell in only six days. In order to make clear his determination to win at all costs, Stalin had the three men in charge of the troops defending Minsk executed for their failure to hold their position. This move, along with unspeakable atrocities by the German soldiers against the people of Minsk, solidified the Soviet will. In the future, Russian soldiers would fight to the death rather than surrender, and in July, Stalin exhorted the nation, "It is time to finish retreating. Not one step back! Such should now be our main slogan. ... Henceforth the solid law of discipline for each commander, Red Army soldier, and commissar should be the requirement - not a single step back without order from higher command."
In beginning the start of the fighting on the Eastern Front, the deadliest part of history's deadliest war, Operation Barbarossa would turn out to be arguably the most fateful choice of World War II.
History in 30: The History of the Soviet Union in World War II provides a quick but comprehensive look at the USSR's experience in history's deadliest war.