Raul Hilberg, a well-known historian of the Holocaust, identified four distinct Phases of the Holocaust.

Economic discrimination and separation
After the Jews were segregated by definition from the rest of the people, and their economic ties with the rest of the society were mostly cut or restricted, the Jews were physically separated from the rest of the society. Either by forcing them to move to special houses or to ghettos, Jews had to live in often desperate conditions.
By concentrating the Jews into ghettos, concentration camps or forced-labor camps, the Jews were completely cut off from the rest of society, completely under the control of the Nazis. The Nazis controlled the amount of food that entered the ghetto and used the population for slave labor.
Isolated from society, without money, and under the control of the Nazis, the Jews were now defenseless.