Established in 1920, the Socialist Zionists (Socialist Zionism meant dealing diplomatically with other nations and exercising restraint militarily) established the Haganah, an illegal underground army, as a result of increasing Arab violence. 

The Haganah was the paramilitary group formed to defend Jews during post-Ottoman British occupied Palestine. It is interesting to note that there was a period when Jewish communities hired local Arabs to protect their populations against roving Bedouin gangs. Unfortunately, over time, many of the Arabs joined together with those who were attacking and thus, the Jews realized they only had themselves for protection. 

The Haganah, meaning defense, had a philosophy of showing restraint and would not initiate any battles with the Arabs. Prior to this, there existed smaller Jewish groups to protect Jewish lands, but they quickly joined forces to become the Haganah, whose leader was David Ben-Gurion, Chairman of the Jewish Agency. The now more robust defense organization would, of course, defend itself against Arab infiltrators. 

Haganah officer1948
In the land of Israel, a female officer of the Haganah
defends the Jewish people; 1948

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The British outlawed the Haganah and made it a crime to obtain weapons for any Jewish organization. The same stringent laws were not imposed upon the Arabs. It became increasingly apparent that Jews could not rely on the British to protect them against Arab hostilities, and that they would need to defend themselves.

This thought process continues until today; Israel only relies on itself for protection – you will not find U.S. troops or any other forces in Israel defending the Jewish homeland. In addition to defense, the Haganah was instrumental in smuggling Jews out of Nazi occupied Europe and North Africa during WWII, and even post-war to ensure safe passage of Jews to Eretz Yisrael (Land of Israel). The Haganah eventually morphed into the Israeli Defense Forces, the current standing army of Israel.

Early Haganah training
During the 1920 riots, smaller Jewish defense groups came together to form the Haganah. This picture shows early days of training.

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Haganah Troops
Haganah soldiers patrolling the area known as "no man's land" in Katamon, Jerusalem; May 6, 1948

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