Soon after the completion of secondary school, he joined the Socialist- Zionist youth movement Hashomer Hatzair, where his organizational and managerial tasks are delegated. At the beginning of 1940 he founded as an activist in the underground youth groups and illegal newspapers. In the summer of 1941 Anielewicz began to transform the youth movements from the ground in an armed resistance movement and to build self-help organizations in the Warsaw ghetto.
In summer 1942 he was in the southwestern part of Poland, was occupied in 1939 by Nazi Germany and was a part of the Polish General. After his return to Warsaw remained from the original 350,000 Jews in the ghetto only 60,000, the rest had been deported to Treblinka for the most part. Anielewicz did everything to strengthen the Jewish Fighting Organization ( Polish Żydowska Organizacja Bojowa, short ŻOB ), and fought the Nazis active. After the mass deportations in the ghetto there was significantly more support for the armed resistance than ever before. In November 1942, Anielewicz was appointed commander of the Jewish Fighting Organization. Until January 1943, some weapons were collected on the Polish side of the city.
On January 18, 1943, the staff of the ZOB was surprised by the second mass deportations from the Warsaw ghetto and had no time to report the situation. Anielewicz commanded a major street battle. Some fighters joined aware of the deportees. At a signal, they attacked the SS guards, while the remaining Jews fled. Many Jewish resistance fighters were killed, but Anielewicz remained alive.
Four days later, the deportations came to a standstill. Since the staff of the ZOB was clear that they could bring the deportations only to a temporary halt, they prepared intensively for the next collision with the SS.
On April 19, 1943, the last deportation of the Jews was launched. This was the signal for the second phase of the Warsaw Ghetto Uprising. First, the Jewish resistance fighters of the SS inflicted losses; the street fighting lasted three days.
At the beginning of the uprising Anielewicz was in the middle of the main group of fighters. When the riots ended and the SS inserting a massive troop numbers are against the few hundred Jewish fighters, he retired with his staff in the bunker in Mila 18. The SS burned the ghetto down block to block. On May 8, 1943 Anielewicz and most of the staff of the ZOB came in the bunker killed.
In a farewell letter to a friend Anielewicz wrote on April 23, 1943 (excerpt):
" Farewell, my friend! Maybe we 'll meet again. Most importantly, that the dream of my life has come true. Jewish self-defense in the ghetto has been realized. Retaliation and resistance from the Jewish side has become a fact. I have been witness to the heroic courage of the Jewish fighters. "
To the Memory of Mordechai Anielewicz dedicated in Israel, among other things, the kibbutz Yad Mordechai, which was founded in 1943. In addition, numerous streets in Israel are named after him and at Yad Vashem is a memorial to him on the Warsaw Ghetto Square. Also in Warsaw carries a street its name; and at his birthplace there is a monument with a Polish, Hebrew and English words.