In June 1941 Germany invaded the Soviet Union. Following the army were special mobile killing units known as Einsatzgruppen. They rounded up Jews and murdered them. More than one million Jews and millions of Soviet citizens were killed. In January 1942 Nazi leaders decided to kill all Jews living in the areas occupied by the German army. This project, which became a national priority for Germans, was known as the “Final Solution to the Jewish Problem.” Within a few months, the Germans began to empty the ghettos and force Jews into crowded boxcars. They were transported, without food or water, to death camps. Many died during the trip. When the trains arrived a selection occurred: camp guards killed women, children and the aged with poison gas. Only the able-bodied young remained alive to work as slave laborers. The Germans forced them to carry the victims’ bodies and sort out their belongings. When they were no longer needed, these men and women were killed. In addition to Jews, the Nazis sent other people to concentration camps and slave labor camps. Among these were political prisoners, homosexuals, Jehovah’s Witnesses, persons with various disabilities, and ethnic minorities like Gypsies and persons of color. Many people were killed because they did not work hard enough. Others were tortured and killed for no reason other than racial hatred. Some Jews fought back. They joined partisan units that lived in the forests and attacked Germans. Jews in the Warsaw Ghetto rebelled and fought until the Germans finally burned the ghetto down. In the Treblinka death camp, the Jews rioted and several hundred escaped. At Auschwitz, a Jewish group attacked the guards and destroyed some crematory ovens.