Hashomer was originated by Socialist Zionists,[1] mostly members of Poale Zion, including Israel Shochat, Manya Shochat, Yitzhak Ben-Zvi and Ben-Zvi's wife Rachel Yanait, several of whom had earlier formed a small secret guard society called Bar-Giora, which guarded the Sejera commune (now Ilaniya) and Mes'ha (now Kfar Tavor).

[1] Bar-Giora was founded on September 29, 1907, by Israel Shochat, Alexander Zaïd, Yehezkel Henkin in the apartment of Yitzhak Ben-Zvi which was in Jaffa.

Less than two years later, on April 12, 1909, the Bar-Giora leadership decided at a meeting in Kfar Tavor to disband their organization and create a larger one, Hashomer.

In the autumn of 1911 Manya Shochat wrote, on behalf of Hashomer, to Menachem Ussishkin in Odessa asking for money.

In addition to guarding settlements, Hashomer secretly began developing offensive capabilities, seeing itself as the nucleus of a future Jewish army.

A special clandestine assembly of Hashomer members decided to kill Aref al-Arsan, a Bedouin policeman who assisted the Turks and tortured Jewish prisoners.

[3] During World War I, Hashomer was violently opposed to NILI, a Jewish spy network working for the British in Ottoman Palestine.

When they failed to get NILI to cease operations or to hand over a stash of gold coins they’d received from the British, they made an attempt on the life of Yosef Lishansky, one of its members, managing only to wound him.

[4][5][6] Later the Turks caught Lishansky, and he allegedly told them all he knew under torture, implicating twelve members of Hashomer.

Hashomer was successful in providing defense for settlements throughout the country; though it sometimes aroused the ire of Arab watchmen, who lost their jobs, and of pilferers.

In 1920 it was decided to organize the Haganah, a much broader-based group, to cope with new defense challenges and needs of the growing Jewish community in Palestine.

[8] Professor Gur Alroey [he], Dean of Humanities at the University of Haifa, described the Hashomer as "...illiterate people, chauvinist.