Stalin invaded Poland on 18 September 1939. In the subsequent World War II, Stalin stayed neutral but signed a peace deal with Germany's leader Adolf Hitler. He then led a bloody war after Germany invaded the Soviet Union. And after the end of the war Stalin gained control of all Eastern Europe including part of Germany. There, a series of loyal Marxist-Leninist single-party states were set up, extending his power and determining the Soviet Union's position as a superpower.