Group of Jewish Partisans In The Rudniki Forest photo courtesy of Yad Vashem

Some Jews managed to flee, escape, or otherwise hide from the Nazis. Some took to the woods, and were among the founders of the "partisan" movements, the guerrillas fighting against the Germans from the woods. The Germans did not have endless forces, and they could not go into all the forests to flush everyone out. Certain areas were simply left cordoned off.

Jewish Partisans From Kovno photo courtesy of Yad Vashem

The biggest problem for the Jewish underground was that they did not get Allied support like other nationalist groups. Those groups were getting money, weapons and supplies flown in. In addition, the Jews were not only fighting the Germans, but we were also fighting the locals. There were wild gangs who swore that they would kill any Germans they found...

Partisans Marching Through Swamps photo courtesy of Yad Vashem

and also any Jews. The Jewish resistance was fighting on both ends.