From the moment the Ottoman empire joined forces with the Central Powers in November 1914, the Zionist leaders Ze’ev Jabotinsky and Chaim Weizmann worked to create a Jewish legion to fight alongside the British army to liberate Palestine from the sultan. The idea met with strong opposition from both the government of Herbert Asquith and the Zionist leadership, and resulted only in the short-lived Zion Mule Corps, which fought against the Turks in the failed Gallipoli campaign. But in 1917 London reconsidered, allowing for the formation of Jewish units to be made up primarily of Russian subjects living in Britain. Colin Schindler writes:
Leading Zionists—including Nahum Sokolow, Max Nordau, and Ahad Ha’am—had hitherto opposed the formation of a Jewish military force. In addition to compromising the movement’s neutrality, they feared Turkish reprisals in the fashion that had been visited upon the Armenians—massacre and persecution.
British Zionists such as Harry Sacher and Leon Simon believed that Weizmann had been seduced by Jabotinsky’s “jingoism.” The [British] Zionist Federation indignantly opposed the very idea of a Jewish regiment as did Lord Rothschild, later the recipient of the Balfour Declaration.
The fear that a specifically Jewish regiment would impinge on their loyalty to the British crown affected many communal leaders. . . . The anti-Zionist Edwin Montagu, secretary of state for India, considered himself a patriotic Jewish Briton and vehemently opposed the Balfour Declaration. While the cabinet rebuffed his attempt to prevent any declaration, it did accede to his opposition to a battalion of British Jews. “Friendly alien Jews” was another matter—and such battalions would be added to the Royal Fusiliers. British-born Jews themselves could apply to join or be transferred. The poet Isaac Rosenberg wished to join but was killed in action [on the Western front] before he could do so. . . .
Jews from the UK eventually constituted almost one-third of the five battalions of the Royal Fusiliers—now known to history as the Jewish Legion. It was, however, more the symbolism of a Jewish army than the few minor military clashes in the Middle East in 1918 that impacted on Jews worldwide.
Read more on Jewish Chronicle: https://www.thejc.com/news/news-features/birth-of-a-%EF%AC%81ghting-force-for-zionism-1.442999