In 1914, the Ottoman Empire joined the Central Powers, formally entering into World War I with a surprise attack on Russian positions in the Black Sea. During its invasion into Russian and Iranian territories, the Ottomans carried out massacres on the local Armenian populations, escalating the violence into a genocide that ultimately resulted in the deaths of an estimated 1 million Armenians. Many of the leading figures in the Armenian Revolutionary Federation were among those that were deported and murdered by the Ottoman authorities on April 24, 1915. This led the ARF to coordinate the Armenian resistance, which had its epicenter in the city of Van, where tens of thousands of Armenians successfully resisted the genocidal ambitions of the local Ottoman authorities. When the war drew to a close, many members of the Young Turks movement were assassinated by the ARF during Operation Nemesis.