On September 28, 1907, a group of Poalei Zion members gathered at Yitzhak Ben-Zvi's unfurnished apartment in Jaffa and formed Bar-Giora, a Jewish self-defense organization named for Simon Bar Giora, one of the leaders of the Jewish Revolt against the Romans. The founding members were Israel Shochat, Yitzhak Ben-Zvi, Mendel Portugali, Israel Giladi, Alexander Zaid, Yehezkel Hankin, Yehezkel Nissanov and Moshe Givoni.
The goal of the organization was settling the land and guarding it from Arab attackers. Previously, Arab guards had been hired for protection. Many Jews refused to employ members of Bar-Giora fearing it would cause more friction with the local Arabs.
As a motto, Bar-Giora chose a line from Yaakov Cohen's poem, Habiryonim: "In fire and blood did Judea fall; in blood and fire Judea shall rise." This was one of the mottos of the Jewish defenders during the pogroms in the Russian Empire. Members swore an oath of secrecy, discipline, selfless service, devotion to the cause and loyalty. All decisions had to be ratified by unanimous vote. All members were required to have least a year's experience in farming. Guarding was put off until the members of the organization had gained enough experience and knowledge of the land.
When Hashomer was formed in 1909, Bar-Giora was absorbed into it.