 On his arrival he and Ben Zvi went on a tour of 35 cities in an attempt to raise a pioneer army, Hechalutz, of 10,000 men to fight on Turkey's side.After the Balfour Declaration of November 1917, the situation changed dramatically and in 1918 Ben-Gurion, with the interest of Zionism in mind, switched sides and joined the newly formed Jewish Legion of the British Army.He volunteered for the 38th Battalion, Royal Fusiliers, one of the four which constituted the Jewish Legion.