Before World War II, Warsaw was the center of Jewish life and culture in Poland. Warsaw's prewar Jewish population of more than 350,000 constituted roughly 30 percent of the city's total population. The Warsaw Jewish community was the largest in Europe and second to only New York City worldwide.
On September 29, 1939, the German military occupied Warsaw. In October 1940, the Germans ordered the establishment of a ghetto into which all Jewish residents were ordered to live. The ghetto, enclosed by a 10 foot wall topped with barbed wire and closely guarded to prevent illegal movement, was sealed off from the rest of the city in November 1940.
Sources: U.S. Holocaust Memorial Museum