Begin quickly made a name for himself as a fierce critic of the dominant Zionist leadership for being too cooperative with the British, and argued that the only way to save the Jews of Europe, who were facing extermination, was to compel the British to leave so that a Jewish state could be established. In 1942 he joined the Irgun ("Etzel"), an underground Zionist paramilitary organization which had split from the main Jewish military organization, the Haganah, in 1931. Begin assumed the Irgun's leadership in 1944, determined to force the British government to remove its troops entirely from Palestine. The official Jewish leadership institutions in Palestine, the Jewish Agency and Jewish National Council ("Vaad Leumi"), backed up by their military arm, the Haganah, had refrained from directly challenging British authority. They were convinced that the British would establish a Jewish state after the war due to support for the Zionist cause among both the Conservative and Labour parties. Giving as reasons that the British had reneged on the promises given in the Balfour Declaration and that the White Paper of 1939 restricting Jewish immigration was an escalation of their pro-Arab policy, he decided to break with the official institutions and launch an armed rebellion against British rule, in cooperation with Lehi, another breakaway Zionist group.