Already between September 1939, when Germany invaded Poland, and June 1941, the beginning of Operation Barbarossa (the war between Germany and the Soviet Union) mass executions took place on a large scale. Hundreds of thousands of Poles, Ukrainians, Lithuanians, and Russian Jews were victims of these mass executions. Many other citizens, such as the non-Jewish Polish intelligentsia, were murdered. A small number of mobile units, known as the Einsatzgruppen (task forces), carried out these mass executions. These units consisted of SS and policemen, who could be deployed on short notice for ‘special assignments'. Deployment of the Einsatzgruppen was organised by the Reichssicherheitshauptambt under leadership of Reinhard Heydrich. They were often assisted by the Ordnungspolizei (German public order police), the local police, and the local population. Commandos of the Einsatzgruppen often gathered half or even the entire population of a village, led them away, shot them, and buried the bodies in mass graves or open pits. Starting in the autumn of 1941, prisoners were also gassed in hermetically sealed lorries. Deployment of the Einsatzgruppen was the first step towards the Endlösung (final solution) to the Jewish question.