In the early stages of World War I, in 1915 Russian armies advanced on Turkey from the north and the British attempted an invasion from the Mediterranean. Citing the threat of internal rebellion, the Ottoman government ordered large-scale roundups, deportations, and systematic torture and murder of Armenians beginning in the spring of 1915. Estimates vary from 600,000 to 2 million deaths out of the prewar population of about 3 million Armenians. By 1917 fewer than 200,000 Armenians remained in Turkey.
Whatever the exact dimensions of the genocide, Armenians suffered a demographic disaster that shifted the center of the Armenian population from the heartland of historical Armenia to the relatively safer eastern regions held by the Russians. Tens of thousands of refugees fled to the Caucasus with the retreating Russian armies, and the cities of Baku and Tbilisi filled with Armenians from Turkey. Ethnic tensions rose in Transcaucasia as the new immigrants added to the pressures on the limited resources of the collapsing Russian Empire.