The Warsaw Ghetto was the largest of the Jewish ghettos established by the Nazis , holding an estimated 445,000 Jews when it was established in 1940. The ghetto covered a small portion of Warsaw (2.4 percent of the city’s area), but held about a third of the city’s population. The Jewish residents suffered through overcrowding, meager food rations, disease, and deportations to concentration camps.In July 1942, the Nazis began deporting ghetto residents to concentration camps . According to the United States Holocaust Memorial Museum, approximately 300,000 Warsaw Jews were killed or deported between July and September; about 35,000 were allowed to stay and about 20,000 remained in hiding, but “deportation seemed inevitable,” according to the museum.Several resistance movements were formed within the ghetto, most prominently the Jewish Combat Organization (ZOB) and the Jewish Military Union (ZZW). The movements were united under the leadership of 24-year-old Mordechai Anielewicz.On Jan. 18, 1943, the Warsaw ghetto launched its first act of resistance, attacking SS officers making a surprise deportation of 8,000 Jews. Though the Jewish fighters suffered significant losses, they killed 20 Germans and held the number of deported Jews to 5,000. The Nazis decided to temporarily halt deportations.Over the next several months, the ZOB and ZZW built bunkers and tunnels, and smuggled in weapons from the Polish Home Army. They were prepared for war on April 19, when the Nazi SS attempted liquidate the ghetto, a task it believed would take three days.