Literally, defense. Abbreviation for Irgun HaHaganah, the Jewish defense organization formed in 1919-20 by volunteers in early Jewish communities as home guards for protection against hostile bands. It became the military arm of the Jewish Agency (q.v.) and went underground during the British Palestine Mandate period (1922-48) when it was declared illegal. Along with the Jewish Brigade, which fought with the Allied forces in World War II, it formed the nucleus of the Israel Defense Forces (IDF) established in 1948.