This chapter demonstrates the effect of the mobilization of ideas and manpower on the Zionist movement during the two world wars as well as a smaller international conflict that adumbrated World War II. During World War I, the Zionist movement sponsored the formation of Jewish units for the British armed forces, and although these units' military accomplishments were modest, they had a galvanizing effect on Jewish collective solidarity throughout the western world. A very different type of international mobilization sent thousands of Jews into the International Brigades in the Spanish Civil War. Ideologically, these wars were perceived as serving Jewish interests, albeit often conflicting ones such as Zionism, on the one hand, and international socialism, on the other. Operationally, these were, for Jews, international conflicts, involving mass movements of Jews not only as refugees or inducted soldiers but also as volunteer fighters.