During the Holocaust, Betar members revolted numerous times against the Nazis in occupied Europe. The largest of these revolts was the Warsaw Ghetto Uprising, in which an armed underground organization fought, formed by Betar and Hatzoar and known as the "Żydowski Związek Wojskowy (ŻZW)" (Jewish Military Union). Despite its political origins, the ŻZW accepted members without regard to political affiliation, and had contacts established before the war with elements of the Polish military. Because of differences over objectives and strategy, the ŻZW was unable to form a common front with the mainstream ghetto fighters of the Żydowska Organizacja Bojowa, and fought independently under the military leadership of Paweł Frenkiel and the political leadership of Dawid Wdowiński.