Haganah (Hebrew: הַהֲגָנָה ha-Haganah, lit. 'The Defense') was the main Zionist paramilitary organization that operated for the Yishuv in the British Mandate for Palestine. It was founded in 1920 to defend the Yishuv's presence in the region, and was formally disbanded in 1948, when it became the core force integrated into the Israel Defense Forces shortly after the Israeli Declaration of Independence.

Formed out of previous existing militias, Haganah's original purpose was to defend Jewish settlements against Arab attacks; this was the case during the 1921 Jaffa riots, the 1929 Palestine riots, the 1936 Jaffa riots, and the 1936–1939 Arab revolt in Palestine, among others. The paramilitary was under the control of the Jewish Agency, the official governmental body in charge of Palestine's Jewish community during the British era. Until the end of World War II, Haganah's activities were moderate, in accordance with the strategic policy of havlagah (lit. 'self-restraint'), which caused the breakaway of the more radical paramilitaries: Irgun and Lehi. Haganah militants received clandestine military support from Poland and sought cooperation with the United Kingdom in the event of an Axis-led invasion of Palestine through North Africa, prompting the creation of the Palmach, their elite fighting force, in 1941.

Following the end of World War II, the British refused to lift the restrictions on Jewish immigration that they had imposed with the 1939 White Paper. This resulted in Haganah leading a Jewish insurgency against the British authorities in Palestine; the campaign included the paramilitaries' bombing of bridges, railways, and ships used to deport illegal Jewish immigrants, as well as assisting in bringing more diaspora Jews to Palestine in defiance of British policies. After the adoption of the United Nations Partition Plan for Palestine in 1947, Haganah came into the open as the biggest fighting force among the Palestinian Jews, successfully overcoming Arab militias during the Palestinian civil war. After the Declaration of Independence of the State of Israel, the Haganah became the new nation's military force. Shortly after Israeli independence and the beginning of the 1948 Arab–Israeli War, the Haganah was merged with other paramilitary groups and reorganized into the Israel Defense Forces (IDF).

The evolution of Jewish defense organizations in Palestine and later Israel went from small self-defense groups active during Ottoman rule, to ever-larger and more sophisticated ones during the British Mandate, leading through the Haganah to the national army of Israel, the IDF. The evolution went step by step from Bar-Giora, to Hashomer, to Haganah, to IDF.^{[citation needed]}

The Jewish paramilitary organizations in the New Yishuv (the Zionist enterprise in Palestine) started with the Second Aliyah (1904 to 1914).^{[citation needed]} The first such organization was Bar-Giora, founded in September 1907. It consisted of a small group of Jewish immigrants who guarded settlements for an annual fee.^{[citation needed]} It was converted to Hashomer (Hebrew: השומר; "The Watchman") in April 1909, which operated until the British Mandate of Palestine came into being in 1920. Hashomer was an elitist organization which never had more than 100 members. During World War I, the forerunners of the Haganah/IDF were the Zion Mule Corps and the Jewish Legion, both of which were part of the British Army. After the Arab riots against Jews in April 1920, the Yishuv's leadership saw the need to create a nationwide underground defense organization, and the Haganah was founded in June of the same year. The Haganah became a full-scale defense force after the 1936–1939 Arab revolt in Palestine with an organized structure, consisting of three main units—the Field Corps, Guard Corps, and the Palmach strike force. During World War II the successor to the Jewish Legion of World War I was the Jewish Brigade, which was joined by many Haganah fighters. During the 1947–48 civil war between the Arab and Jewish communities in what was still Mandatory Palestine, a reorganized Haganah managed to defend or wrestle most of the territory it was ordered to hold or capture. At the beginning of the ensuing 1948–49 full-scale conventional war against regular Arab armies, the Haganah was reorganized to become the core of the new Israel Defense Forces.^{[citation needed]}

After the 1920 Nebi Musa riots and 1921 Jaffa riots by Palestinians, the Jewish leadership in Palestine believed that the British, to whom the League of Nations had given a mandate over Palestine in 1920, had no desire to confront local Arab groups that frequently attacked Palestinian Jews. Believing that they could not rely on the British administration for protection from these gangs, the Jewish leadership created the Haganah to protect Jewish farms and kibbutzim. The first head of the Haganah was a 28-year-old named Yosef Hecht [he], a veteran of the Jewish Legion.

In addition to guarding Jewish communities, the role of the Haganah was to warn the residents of and repel attacks by Palestinians. In the period between 1920 and 1929, the Haganah lacked a strong central authority or coordination. Haganah "units" were very localized and poorly armed: they consisted mainly of Jewish farmers who took turns guarding their farms or their kibbutzim.^{[citation needed]}

Following the 1929 Palestine riots, the Haganah's role changed dramatically. It became a much larger organization encompassing nearly all the youth and adults in the Jewish settlements, as well as thousands of members from the cities. It also acquired foreign arms and began to develop workshops to create grenades and simple military equipment, transforming from an untrained militia to a capable underground army.^{[citation needed]}