On September 1, 1939, the invasion of Poland by the Germans started and brought some 2 million additional Jews under German control. The Germans needed a place to put the Jews so they developed ghettos to put them in. Ghettos were self-contained areas, usually surrounded by a fence, wall, or armed guards where Jews were forced to live. One of the most famous ghettos that were established was the Warsaw ghetto. 400,000 Jews were placed in here.