With the outbreak of the First World War in 1914, the Caucasus became a major theatre, with the Russian and Ottoman Empires fighting each other in the region. The Russians won several battles and invaded deep into Ottoman territory. However they were concerned that the local population, who were mostly Muslims, would continue to follow the Ottoman Sultan and disrupt the Russian forces, as he was also the caliph, the spiritual leader of Islam. Both sides also wanted to use the Armenian population, who lived across the border, to their advantage and foment uprisings. However, after military defeats, the Ottoman government turned against the Armenians, launching the Armenian Genocide by 1915, in which an estimated 0.8 to 1.5 million Armenians were killed.