National Military Organization was a Jewish underground armed organization founded in Jerusalem in the spring of 1931 by a group of Haganah commanders, headed by Avraham Tehomi, who had left the Haganah in protest against its defensive character. Joining forces with a clandestine armed group of Betar members from Tel Aviv, they formed a parallel, more activist defense organization. In April 1937, during the Arab riots, the organization split over the question of how to react against Arab terrorism, and about half its three thousand members returned to the Haganah, which was controlled by the Jewish Agency . The rest formed a new Irgun Ẓeva'i Le'ummi, which was ideologically linked with the Revisionist movement and accepted the authority of its leader, Vladimir Jabotinsky . Rejecting the "restraint" (Heb. havlagah) policy of the Jewish Agency and the Haganah, the organization carried out armed reprisals against Arabs, which were condemned by the Jewish Agency as "blemishing the moral achievements of the Jews of Ereẓ Israel, hindering the political struggle, and undermining security." Many members and sympathizers were arrested and one of them, Shelomo Ben-Yosef , was hanged for shooting at an Arab bus, but IẒL intensified its activities. It also cooperated with the Revisionist movement in "illegal" immigration , succeeding in smuggling many thousands of Jews into Palestine.

After the publication of the White Paper in May 1939, IẒL directed its activities against the British Mandatory authorities, sabotaging government property and attacking security officers. The British retaliated with widespread arrests, and at the outbreak of World War II, when hundreds of Revisionists and members of IẒL (including its commander David Raziel and his staff commanders) were in prison, IẒL declared a truce, which led to a second split (June 1940) and the formation of a new underground group (Loḥamei Ḥerut Israel , or Leḥi) led by Avraham Stern . IẒL members contributed to the war effort against the Nazis by joining the British Army's Palestinian units and later the Jewish Brigade. During a clandestine operation by an IẒL unit, in cooperation with British Intelligence, against the pro-Nazi regime of Rashid Ali in Iraq, David Raziel fell at Habbaniya, near Baghdad, on May 20, 1941. Ya'akov Meridor took command, and was succeeded in December 1943 by Menahem Begin . By this time, the full extent of the Holocaust in the Nazi-occupied territories had become known, and in February 1944 IẒL declared war against the British administration, which continued to implement the White Paper. It attacked and blew up government offices, several CID headquarters, and four police stations, also capturing weapons and ammunition.

The British authorities made many arrests, and 251 prisoners (including Leḥi members) were deported to Eritrea on Oct. 20, 1944. No organized reaction to the deportation was possible because of the repercussions following the assassination of Lord Moyne by Leḥi in Cairo (Nov. 6, 1944). The Jewish Agency and the Haganah moved against the IẒL in a campaign nicknamed by the underground the "saison" ("hunting season"), during which some of IẒL's members (including several leaders) were kidnapped and handed over to the British authorities. The "saison" limited the scope of IẒL's activities, but did not halt them; after the war it began attacking military installations, bridges, and the vital Kirkuk-Haifa oil pipeline (May 25, 1945). When the British Labour government's anti-Zionist policy disappointed post-war hopes, Haganah, IẒL, and Leḥi formed a united front, sabotaging bridges, railways, and patrol boats. IẒL again attacked CID and police stations, as well as seven army camps, gaining control of their ammunition stores, and damaged planes at two military airfields. The IẒL attacks culminated in blowing up a wing of the King David Hotel in Jerusalem, headquarters of the Palestine government and the military command, on July 22, 1946.

The united fighting front disintegrated in August 1946, after the arrest of the Jewish Agency leaders, but IẒL and Leḥi continued their attacks on military and governmental objectives. The British increased their military strength to a hundred thousand men and reacted with increased ferocity: curfews, arrests, deportations, floggings, and hangings. IẒL reacted by flogging British officers and kidnapping hostages. It also extended its activities abroad, the most striking act being the bombing of the British embassy in Rome on Oct. 31, 1946. Four members of IẒL – Dov Gruner, Yeḥiel Drezner, Mordekhai Alkaḥi, and Eliezer Kashani – were hanged in Acre prison on April 16, 1947, and another two – Meir Feinstein and the Leḥi member Moshe Barazani – who were due to be hanged in Jerusalem, blew themselves up in the condemned cell on April 27. IẒL broke into the fortress at Acre on May 4, and freed 41 IẒL and Leḥi prisoners. Under the pressure of the continual attacks, the British retreated to security zones where they lived in a state of siege. When three other IẒL members, Meir Nakar, Ya'akov Weiss, and Avshalom Ḥaviv, were condemned to death by the British, IẒL kidnapped two British sergeants and hanged them in July, when the three were executed. The IẒL revolt was given wide publicity in the United States, where the Hebrew Committee for National Liberation, led by Peter Bergson (Hillel Kook), was established. In Palestine publicity was conducted through a clandestine radio station, newspapers, and leaflets.