Haganah (Hebrew: הַהֲגָנָה, lit. The Defence) was the main Zionist paramilitary organization of the Jewish population ("Yishuv") in Mandatory Palestine between 1920 and its disestablishment in 1948, when it became the core of the Israel Defense Forces (IDF).

Formed out of previous existing militias, its original purpose was to defend Jewish settlements from Arab attacks, such as the riots of 1920, 1921, 1929 and during the 1936–1939 Arab revolt in Palestine. It was under the control of the Jewish Agency, the official governmental body in charge of Palestine's Jewish community during the British Mandate. Until the end of the Second World War, Haganah's activities were moderate, in accordance with the policy of havlaga ("self-restraint"), which caused the splitting of the more radical Irgun and Lehi. The group received clandestine military support from Poland. Haganah sought cooperation with the British in the event of an Axis invasion of Palestine through North Africa, prompting the creation of the Palmach task force in 1941.

With the end of World War II and British refusal to cancel the 1939 White Paper's restrictions on Jewish immigration, the Haganah turned to sabotage activities against the British authorities, including bombing bridges, rail lines, and ships used to deport illegal Jewish immigrants, as well as assisting in bringing Jews to Palestine in defiance of British policy. After the United Nations adopted a partition plan for Palestine in 1947, the Haganah came into the open as the biggest fighting force among Palestinian Jews, successfully overcoming Arab forces during the civil war. Shortly after Israel's independence declaration and the beginning of the 1948 Arab–Israeli War, the Haganah was merged with other groups and reorganized into the official army of the state.