When reports of these measures leaked out of the Soviet Union, some Western leaders, notably Winston S. Churchill, began to warn that all was not well in "the workers paradise". But most people in the West found such repression unbelievable, so Stalin's crimes were simply ignored, as were Hitler's.
In 1937, Stalin started a Purge of the Red Army, "liquidating" nearly all of the high ranking officers by 1939. This served Stalin's immediate interest by weakening the only group which could have overthrown him, but left the Red Army crippled in the face of the growing threat of war with Germany and Japan.
Stalin supported the Communist party in the Spanish Civil War (1936-1939) against the Fascist "Nationalist" forces supported by Germany and Italy. He was also involved in an undeclared war with Japan, in Asia. From 1936 to 1941, Soviet supplies, advisors and even units of the Red Army were sent to help the Chinese Nationalists. In 1939 and 1940, the Japanese invaded Soviet territory and Japanese and Soviet units fought major battles, lasting months. This was war, in all but name, but Stalin seemed unconcerned.
Hitler had often boasted that he was the enemy of Communism, but in 1939, Stalin concluded the Russo-German Non-Aggression Pact with him. By this Treaty, Hitler would allow the Soviet invasion of Lithuania, Latvia, Estonia and Finland, and would make his Rumanian allies cede their province of Moldavia to the USSR as well. In return, Stalin agreed to divide Poland with Germany. This was not a wise move, for while the USSR would gain more territory than Germany, this removed all the buffer-zones between them and left the Soviets more exposed to attack. Also, by invading these little countries, Stalin lost any chance of an alliance with the British and French to stop further Nazi expansion.
In the 21 months that followed the German invasion of Poland, that marked the official start of War between Germany, France and Britain, Stalin showed a nearly fatal faith in Hitler's promises. He took "his" half of Poland and tried to occupy the other territories "given" to him under the treaty. In Moldavia and the three Baltic Republics, he succeeded, but his brutal methods destroyed any hope that the population would be thankful that he had saved them from the Nazis. In Poland, for example, he had 4,000 of the officers of the defeated Polish Army taken to the Katyn Forest and shot!
The invasion of Finland, which should have been easy for the Red Army, showed how badly the Great Purge had undermined its leadership and morale. Though greatly outnumbered, the Finns gave the Soviet invaders heavy casualties and managed to save most of their country. In the West, Churchill made a speech ridiculing the incompetence of the Red Army, while Sweden and the United States sent modern fighter aircraft to the Finns. But these results did nothing to shake Stalin's confidence. Although officer purges ended, he was utterly unprepared for the Nazi attack to come.
On 21 JUN 41, Hitler launched Operation Barbarossa, the invasion of the Soviet Union. The combination of Red Army disorganization, unreadiness, popular hatred for Stalin and his own military incompetence let the German invaders reach the suburbs of Leningrad and Moscow. They were halted by overstretched supply lines, the beginning of winter, and ZHUKOV, one of the very few generals who had been missed by Stalin's purges.
By the time of the Invasion of Russia, France had been beaten and the British Commonwealth had stood, essentially alone, against Germany and Italy, for a year. Churchill, desperate for any help he could find, began to portray Stalin as Britain's good and faithful ally, Uncle Joe. (Churchill is reported to have said, "If Hitler were to invade Hell, I should have to say a few favorable words about the Devil").
In the winter of '41-42, Japan and the United States entered the war, which helped to save the Soviet Union. First, with Japanese forces needed for war in the Pacific, Stalin felt he could move the forces that had been defending Siberia against Japan eastward, to oppose the Germans. The Siberian troops were experienced and confident from their recent victories in battles with the Japanese, and ideally equipped for a winter campaign against the freezing, thinly-clad Germans.
The United States entry into the war also greatly increased the numbers of planes, trucks, tanks and other supplies being shipped to the Soviets, beyond what Britain had been able to send. These came by air, from Alaska to Siberia; by sea, via the North Atlantic and Barents Sea, to Murmansk; through the Indian Ocean to Iran; and even across the Pacific, where the Japanese allowed Soviet ships to pass through their waters. This aid, arriving when Soviet production had been disrupted by the invasion, was crucial, even though it may not look large when compared to Soviet output over the entire war.
During that winter, with fresh troops and increased supplies, Zhukov pushed the Germans back from the gates of Moscow. In the spring and summer of 1942, Hitler made one of his great blunders, by not finishing the drives on Moscow and Leningrad he had begun the year before. Instead, his panzers were sent southeast, towards Stalingrad and the oil fields at Baku, on the distant Caspian Sea.
Zhukov was sent to Stalingrad, and his successes of 1941 were repeated. The Germans, having made tremendous advances, were halted by over-extended supply lines, cold weather and fierce fighting. When the winter was well underway, Zhukov launched a counteroffensive, as he had in front of Moscow. But this time, he did not merely drive the Germans back. Instead, he trapped the entire Sixth Army behind Russian lines, where it surrendered! After the Battle of Stalingrad, two things were clear. First, the USSR was no longer balanced on the edge of disaster; it could still lose, but it was not going to do so soon. And Zhukov was the man who could save Mother Russia - if Stalin let him do it, for he was also the kind of rival Stalin had murdered so many times before...
Stalin really had no choice. If he left Zhukov in command, the victorious general MIGHT depose him, but if he purged Zhukov, the war would SURELY be lost. Stalin had to keep Zhukov, and, for the rest of the war, in spite of Stalin's titles, it was Zhukov's skill that drove the Germans back to Berlin, and won the war. Stalin was far more fortunate than he deserved, for Zhukov served him faithfully to the end of his life, and Stalin died in bed.