The Great Terror
Stalin ruled with an iron fist and trusted no one. He feared that opposition from high ranking military officials could be a direct a threat to his title. During the 1930's, what is known as The Great Terror or Great Purge, resulted in the death's of million's of Soviet citizens. Over 800,000 ranking military figures were killed and hundreds of thousands of other citizens also perished because of Stalin's unstable paranoia. Stalin installed the NKVD, which was his secret police that carried out most of his ordered killings. Stalin also encouraged citizens to spy on their neighbors and help to inform his regime of any possible treason. His accountability system created an incredible amount of fear across the entire country. By the end of the terror nobody questioned his leadership and any decisions he made. Those who did question his motives, like the kulaks, were killed or sent to the Gulag prison camps.
While this all happened before WWII, it is important because his removal of nearly all high ranking military officers resulted in the severe lack of military leadership and strategic planning in the Red Army. The officers killed were quickly replaced by younger, inexperienced officers that were undoubtably loyal to Stalin. Although, their inadequacies were acknowledged by Hitler, which helped reassured himself of how easy it would be for his German war machine to conquer the Soviet Union.
While this all happened before WWII, it is important because his removal of nearly all high ranking military officers resulted in the severe lack of military leadership and strategic planning in the Red Army. The officers killed were quickly replaced by younger, inexperienced officers that were undoubtably loyal to Stalin. Although, their inadequacies were acknowledged by Hitler, which helped reassured himself of how easy it would be for his German war machine to conquer the Soviet Union.
Soviet Propaganda
Stalin also abused his powers by controlling all media in the Soviet Union. He used propaganda to display himself as an all powerful leader that cared for his people and their salvation. He also used propaganda during World War II, by claiming the Red Army was stronger and suffering less casualties then they actually were. The Soviet and German battles were some of the deadliest battles ever fought in the history of the world. Stalin had renamed cities in his name and even rewrote the Soviet National Anthem to include himself. Furthermore, Stalin had even rewrote history books to claim that he had a larger role in the revolution against Tzar Nicholas II. All of this was done to create a god like image of himself and to further justify to the people of his rightful position as supreme ruler. Stalin ruled with fear and his use of the media made people forget the fact that he killed so many of his own countryman. Ironically, the people of the Soviet Union idolized Stalin and were very supportive of him.
Beginning of World War II
At the start of World War II, the Red Army was in shambles, they were no match for the well disciplined German army. As a direct result of Stalin's purging of all potentially disloyal Soviet officers, the Red Army was inexperience, lacked leadership, and unorganized. Despite his own doings, Stalin recognized his army's weaknesses, which is why he initially signed a non-aggression pact with Hitler in 1939. Their allegiance served multiple purposes for both parties. It allowed Germany to temporarily avoid fighting a two front war, as well as provide trade agreements for necessary resources to support their causes. For Stalin, it bought him time to build his army and to prepare for an inevitable war with Germany. Their pact as well gave him rights to parts of Poland, which he had long wanted. He also benefited from the trade agreement, which provided armature for Soviet resources. Beforehand, Britain and France had considered forming an alliance with Stalin in efforts to deter Hitler from invading Poland. Both Britain and France, desperately did not want to enter a second war with Germany because they were still in the beginning stages of recovery form World War I. While a pact with Stalin would of outnumber the Germans, Churchill could not bring himself to trust Stalin.
German Invasion
June 1941, Hitler broke the non-aggression pact by advancing into Soviet territory. Despite numerous warnings from Soviet spies and western leadership, Stalin's failed to properly secure Soviet borders from an invasion. Quickly, Germany's blitzkrieg pushed through the Ukraine and headed towards Moscow. Initially, Stalin had feared the attack so much, that he retreated into hiding for several days, but eventually came out from hiding, and declared a state of emergency to defend Moscow from a foreign invasion. Stalin gave speeches calling for all Soviet's to help protect their motherlands from an invading Hitler and asked that his country unite together to help in their defenses. Military divisions were pulled from eastern fronts and civilian volunteers helped defeat Germany's first invasion. Though the Soviets lacked skilled fighter, they had an abundance of manpower. Further misplannings by the Germans regarding supplies and conditions hindered the German advance. Many lives were lost during the battles with Germany, but Stalin bared little remorse for putting millions of peoples lives at risk. Stalin was an incompetent military tactical leader and could be held responsible for a large portion of his armies losses. He often refused to allow troops to retreat from their positions and commanded them to fight to the death. Often, if a soldier of the Red Army was taken prisoner and released they were killed because Stalin believed they had given up information to the German's.
The Battle of Stalingrad
In 1942, Germany launched a second attack on the Soviet Union in the direction of Stalingrad and the Caucus oil fields. The Battle of Stalingrad, was the turning point of World War II, which greatly damaged both Germany's military and morale. Stalingrad was the bloodiest battle during all of World War II, resulting in one million Soviet deaths and over a million and a half Axis casualties. As the Germans approached Stalingrad, a victory seemed most likely in their favor. Although, it was the sheer determination and desperation of the Soviet troops, that was responsible for defeating the Germans. The Soviets were outnumbered and out skilled, but they outfought the German's for five months and successfully defended their city. The battle marked the beginning of the end of Hitler's reign in Germany. Stalin played an important role in motivating his nation to dig in and fight for their country. The Battle of Stalingrad was a great success for Stalin and the Soviet Union despite such a horrific number of deaths.
Stalin and the Allies
Before the War, Allied Nations did not trust Stalin and were wary of his emerging economic and military powers. Even after his defeat of the Germans at Stalingrad, tension between him and western powers remained high and carried on post war into the Cold War. Stalin insisted that Allied nations assist the Soviet Union by attacking Germany from the western front. Allied nations were very resistant to collaborating with Stalin and withheld their troops from advancing into German territories. The Allies were not happy with Stalin's original pact with Hitler and did not trust the communist party and their intentions, but they felt that Germany and Japan were currently a much greater threat to world peace. Despite knowing Stalin was just as evil as Hitler, The United States and Britain did join forces with Stalin and all parties were large factors in the defeat of Nazi Germany. Roosevelt's main reasoning for giving into helping Stalin, was to prevent a second alliance between Hitler and Stalin. Furthermore, Roosevelt felt that he could help the Soviet Union and help develop a "more democratic socialism." Though Stalin was guilty of mass ethnic cleansing and corruption, he was able to continuously take attention off of himself and put it back on Nazi Germany. Ultimately, Stalin's Red Army successfully defeated Germany in Stalingrad and helped in the final defeat of Berlin, Germany. After the defeat of Hitler, tension rose high again between a stubborn Stalin and the Allied Nations. While the Allies were trying to reinstate governments in conquered countries, Stalin refused to release his new territories and instead adopted them as Soviet satellite states. As well as attempted to declare Berlin as part of the Soviet Union. The Allies refused to allow Stalin to take Berlin and from then out had turned their attentions towards Stalin and the USSR, thus marking the beginning of the Cold War.
Sources
Stalin's Curse: Battling for Communism in War and Cold War by Jacob Sherman gave a deep account of Stalin's relationships with the Allies. It also explained the Great Terror as well as events leading into the Cold War.
This biography does a great job explaining Stalin's impact in World War II. It also explains Stalin's delicate relationship with the Allies.
This Magazine article by Geoffrey Roberts explains in depth the circumstances surrounding the Battle of Stalingrad.
The Export of Terror article by Claudia Weber explains the Great Terror as well other mass murder's by Stalin.