Marshal Joseph Stalin, Premier of the Soviet Union. In the 20 years between the Two World Wars, Stalin gained undisputed leadership of the Communist Party by murdering all of the other leaders of the Russian Revolution who had once shared power with him. He maintained control of the Soviet Union by a ruthless reign of terror in which millions of its citizens were arrested and condemned to death, or almost certain death in Siberian labor camps. Most of these arrests were made in a series of organized "Purges" targeted at specific groups or institutions in society, e.g., engineers, teachers, or former landowners; but many of these arrests were made entirely at random. The lesson being that no one would be safe from the Secret Police. These policies produced great resentment, but succeeded in terrorizing the population into submission.
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.