World War I (1914–1918)

The Young Turk government had signed a secret treaty with Germany and established the Ottoman-German Alliance in August 1914, aimed against the common Russian enemy but aligning the Empire with the German side. There were several important Ottoman victories in the early years of the war, such as the Battle of Gallipoli and the Siege of Kut, but there were setbacks as well, such as the disastrous Caucasus Campaign against the Russians.

In 1915, the Russian Army advanced in eastern Anatolia with the help of Armenians from the Caucasus, and aided by some Ottoman Armenians, the Empire issued the Tehcir Law, which deported Armenians resulting in the Armenian Genocide. Using forced marches and massacres, the Armenians living in eastern Anatolia were uprooted from their ancestral homelands and sent southwards to Syria and Mesopotamia. Estimates vary on how many perished but scholars give figures ranging from 300,000 (per today’s Turkey), 600,000 (per early estimates by Western researchers) up to 1.5 million (per today’s Western and Armenian scholars).

The Arab Revolt from 1916 on turned against the Ottomans in the Middle East. When the Armistice was signed on October 30, 1918, the only parts of the Arabian peninsula still under Ottoman control were Yemen, Asir, Medina, northern Syria and northern Iraq. These territories were handed over to the British. The Ottomans were ordered to evacuate present-day Georgia, Armenia and Azerbaijan, which they had taken over after Russia's retreat in 1917.