In 1909 the Shomrim were formally organized throughout the area of Jewish settlement in Palestine and renamed HaShomer, or the Watchmen. Although HaShomer numbered fewer than 100 men at the organization's peak, these armed militias became extremely important to Israeli military tradition. Many members of HaShomer joined the Jewish Legion, which fought with the British against imperial Germany during World War I. They also established a precedent of armed self-defense of the Zionist movement, which during the War of Independence in 1948 would flower into the IDF. .
Increasing tensions between the Arab communities and the growing Jewish communities of Palestine brought the need to expand the capacity of the Yishuv for self-defense. In 1920, after serious Arab disturbances in Jerusalem and in northern Palestine, HaShomer militias were disbanded and replaced by the Haganah (abbreviation for Irgun HaHaganah, Defense Organization), which was intended to be a larger and more wide-ranging organization for the defense of all Palestinian Jewry. By 1948, when it was disbanded so that the IDF would be the sole Israeli military organization, the Haganah was a force of about 30,000. .
The Haganah, financed originally through the Zionist General Federation of Laborers in the Land of Israel (HaHistadrut HaKlalit shel HaOvdim B'Eretz Yisrael, known as Histadrut) and later through the Jewish Agency, operated clandestinely under the British Mandate, which declared the bearing of unauthorized arms by Palestinian Jews to be illegal. Arms and ammunition were smuggled into the country, and training was conducted in secret. In addition to guarding settlements, the Haganah manufactured arms, built a series of roads and stockades throughout Palestine to facilitate defense, and organized and defended groups of Jewish immigrants during periods under the Mandate when immigration was illegal or restricted. .
Arab attacks on Jewish communities in 1921 and 1929 found the Haganah ill equipped and ineffective: more than 100 Jews were killed in 1929 alone.
