Between 1915 and 1917, Russia occupied virtually the entire Armenian part of the Ottoman Empire. Then in October 1917, the Bolshevik victory in Russia ended that country's involvement in World War I, and Russian troops left the Caucasus. In the vacuum that remained, the Armenians first joined a Transcaucasian federation with Azerbaijan and Georgia, both of which, however, soon proved to be unreliable partners. The danger posed by the territorial ambitions of the Ottoman Turks and the Azerbaijanis finally united the Caucasian Armenian population in support of the Armenian Revolutionary Federation ( also known as the Dashnak, a shortened form of its Armenian name) program for autonomy. In May 1918, an independent Armenian republic was declared; its armies continued to fight on the Allied side south of the Caucasus until the Ottoman Empire surrendered in October 1918. The independent republic endured from May 1918 to December 1920. In the new government, Dashnak leaders R.I. Kachazuni and A.I. Khatisian became prime minister and foreign minister, respectively.
The Republic of Armenia included the northeastern part of present-day eastern Turkey, west along the Black Sea coast past Trabzon and southwest past Lake Van. But Armenia's precarious independence was threatened from within by the terrible economic conditions that followed the war in the former Ottoman Empire and by the territorial ambitions of Soviet Russia and the nationalist Turks under Kemal Atatürk. Atatürk had rehabilitated Turkey rapidly under a new democratic system, but the ruling party still hoped to create a larger state by taking territory in western Armenia from which Armenians had been driven. Resurgent Turkish forces recaptured Armenia (autumn), taking Kars (October 21, 1919) and once again massacring a large number of Armenians. In defending its independence, the Republic of Armenia waited in vain, however, for the material and military aid promised at the Paris Peace Conference in 1919. The Allies' memories of the 1915 massacre faded as war weariness and isolationism dominated their foreign policy.
In agreeing to the 1920 Treaty of Sèvres, the World War I Allies and Turkey recognized Armenian independence; as part of the treaty, Armenia received some disputed territory in what had been the Ottoman Empire. However, most of western Armenia remained in Turkish hands. Eastern Armenia, ravaged by warfare, migration, and disease, had an Armenian population of only 720,000 by 1920. Caught between the advancing Turks and the Red Army, which had already occupied neighboring Azerbaijan, in November 1920 the Dashnak government made a political agreement with the communists to enter a coalition government. The Treaty of Aleksandropol', signed by this government with Turkey, returned Armenia's northern Kars District to Russia and repudiated the existence of Armenian populations in newly expanded Turkey.