Mordechai Anielewicz was born to a poor Polish Jewish family in Warsaw in 1919. He joined the Zionist youth movement "Hashomer Hatzair" (the Young Guard), and became a youth leader and organiser.

On the 7th September 1939, Mordechai Anielewicz and his friends left Warsaw planning to return after the Polish army faught off the German invasion. However, the Soviet army invaded from the East under the terms of the Molotov-Ribbentrop Pact. Mordechai tried to escape to Romania, and was taken prisoner by the Russians, but eventually released.

He returned to Warsaw but left again for Vilna (also known as Vilnius, then under Soviet control) to secure help from the Jewish underground there.

Mordechai and his girlfriend Mira Fukrer returned to Warsaw, with others. They became full-time underground resistance members, organising meetings, leaflets, and activities throughout Poland.

After the Final Solution intensified and Mordechai heard about the mass murder of the Jews of Europe, he began to organise his network of educators and organisers into a fighting force. But he was away from Warsaw when, in the summer of 1942, the first major deportations from the Warsaw ghetto begun; he returned to find that of the 350,000 residents of the ghetto, only 60,000 were left. He actively recruited for the ZOB (Zydowska Organizacja Bojowa, Jewish Fighting Organization) and encouraged most of the disparate movements to join it. In November 1942 he was elected as its supreme commander.

In January 1943 the Nazis arrived to conduct another deportation. Mordechai Anielewicz led the resistance in their efforts to fight off the German army, and even though many of the members of Mordechai's own youth group died in the fighting, they were able to force the Nazis to postpone the operation.

On April 19th The Nazis returned to finish the job. The Warsaw Ghetto resisted for a month, but to no avail.

On April 23rd, he wrote in one of his last letters, to a friend:

Be well, my friend! Perhaps we will see one another again. The most important thing is that my life's dream has come true. Jewish self-defense in the ghetto has been realized. Jewish retaliation and resistance has become a fact. I have been witness to the magnificent heroic battle of the Jewish fighters.

Mordechai Anielewicz died aged only 23 on the 8th of May 1943, in his bunker at Mila 18, after a 2-hour Nazi assault.


Sources:
http://fcit.coedu.usf.edu/holocaust/people/Anielewi.htm
http://www.us-israel.org/jsource/biography/Anielevich.html