After he obtained an official vacation from the Polish army, Begin accepted the appointment as Commander of the Irgun. Over four years he lead the underground organization, and throughout that period was pursued by the British police and by the Jewish leadership of the Yishuv. Persistent threats forced the Begin Family to move between hiding places, and forced Begin to take on false identities, such as ‘Rabbi Israel Sassover’ and ‘Yonah Konigshofer’.
On February 1, 1944, Begin issued a declaration of uprising on behalf of the Irgun, against the British rule in Israel, demanding it hand over its power immediately to a temporary Hebrew Government. Two weeks after the announcement, Irgun fighters detonated a bomb at the British immigration departments in Jerusalem, Haifa and Tel Aviv, and two more weeks later blew up the income tax offices those three cities. A month later they attacked the National British Headquarters and the secret police headquarters in Jerusalem, destroyed the secret police headquarters in Jaffa and blew up the secret police headquarters in Haifa.