He left Warsaw on 6 September 1939 and made his way north-east towards Wolkowysk. He was caught between the invaders, the Germans in the west, and the Soviets who then approached from the east. Poland's independence was extinguished by terms of the pact between Stalin and Hitler. He barely evaded German capture, and traveled through Lithuania to reach Sweden by early spring of 1940. There he lectured at the University of Stockholm. Curious about the manner of imposition of Nazi rule he started to gather Nazi decrees and ordinances, believing official documents often reflected underlying objectives without stating them explicitly. He spent much time in the central library of Stockholm, gathering, translating and analysing the documents he collected, looking for patterns of German behaviour. Lemkin's work led him to see the wholesale destruction of the nations over which Germans took control as an overall aim. Some documents Lemkin analysed had been signed by Hitler, implementing ideas of "Mein Kampf" on "Lebensraum", new living space to be inhabited by Germans. With the help of his pre-war associate McDermott, Lemkin received permission to enter the United States, arriving in 1941.