After the Invasion of Poland (1 September 1939), by Nazi Germany and the Soviet union, in occupied Poland each side proceeded to eliminate any possible resistance leader. In their part of occupied Poland, the Nazis began the Second World War (1939–45) with the extermination of the Polish intelligentsia, by way of the military operations of the Special Prosecution Book-Poland, the German AB-Aktion in Poland, the Intelligenzaktion, and the Intelligenzaktion Pommern. In their part of occupied Poland, the Soviet Union proceeded with the extermination of the Polish intelligentsia with operations such as the Katyn massacre (1940), during which university professors, physicians, lawyers, engineers, teachers, writers and journalists were murdered.