Ghettos were established by Nazi Germany in hundreds of locations across occupied Poland after the German invasion of Poland. Most ghettos were established between October 1939 and July 1942 in order to confine and segregate Poland's Jewish population of about 3.5 million for the purpose of persecution, terror, and exploitation. In smaller towns, ghettos often served as staging points for Jewish slave-labor and mass deportation actions, while in the urban centers they resembled walled-off prison-islands described by some historians as little more than instruments of "slow, passive murder", with dead bodies littering the streets.