When military authorities were in complete control, they applied martial law firmly but reasonably. Captured opponents to their regime were tried as spies. Sometimes they were executed by firing-squads; more often they were imprisoned. The hostage system - the custom of punishing the innocent when the guilty parties could not be found -did much to further tarnish then image. But the military governments were mild compared with the SS, which gradually infiltrated everywhere. 8 Once the invasion of Soviet Russia was under way, the Wehrmacht was forced to hand over the SS responsibility for 'keeping order' behind the front lines. In December 1941, Field-Marshal Keitel issued an enabling decree entitled 'Night and Fog', which authorized the SS to assume arbitrary powers. From that date, summary executions of communists, Jews and real or imagined enemies multiplied. The SS also carried out group massacres, razings of whole villages, pitiless deportations of populations. Their systematic reign of terror was introduced into western Europe in 1943. Persons suspected of committing acts of violence against the occupying forces were tortured.