The Ottoman Empire joined the Central Powers in the war, the secret Ottoman-German Alliance having been signed in August 1914. It threatened Russia's Caucasian territories and Britain's communications with India via the Suez Canal. The British and French opened overseas fronts with the Gallipoli (1915) and Mesopotamian campaigns. In Gallipoli, Turkey successfully repelled the British, French and Australian and New Zealand Army Corps (ANZACs). In Mesopotamia, by contrast, after the disastrous Siege of Kut (1915–16), British Imperial forces reorganized and captured Baghdad in March 1917. Further to the west, in the Sinai and Palestine Campaign, initial British setbacks were overcome when they captured Jerusalem in December 1917. The Egyptian Expeditionary Force, under Field Marshal Edmund Allenby, broke the Ottoman forces at the Battle of Megiddo in September 1918.

Russian armies generally had the best of it in the Caucasus. Enver Pasha, supreme commander of the Turkish armed forces, was ambitious and dreamed of conquering central Asia. He was, however, a poor commander. He launched an offensive against the Russians in the Caucasus in December 1914 with 100,000 troops; insisting on a frontal attack against mountainous Russian positions in winter, he lost 86% of his force at the Battle of Sarikamish.

General Yudenich, the Russian commander from 1915 to 1916, drove the Turks out of most of the southern Caucasus with a string of victories. In 1917, Russian Grand Duke Nicholas assumed command of the Caucasus front. Nicholas planned a railway from Russian Georgia to the conquered territories, so that fresh supplies could be brought up for a new offensive in 1917. However, in March 1917, (February in the pre-revolutionary Russian calendar), the Czar was overthrown in the February Revolution and the Russian Caucasus Army began to fall apart.

The army corps of Armenian volunteer units realigned under the command of General Tovmas Nazarbekian, with Dro as a civilian commissioner of the Administration for Western Armenia. The front line had three main divisions commanded by Movses Silikyan, Andranik, and Mikhail Areshian. Another regular unit was under Colonel Korganian. More than 40,000 men in Armenian partisan guerrilla detachments accompanied the main units.

Instigated by the Arab bureau of the British Foreign and Commonwealth Office, the Arab Revolt was a major cause of the Ottoman Empire's defeat. The revolts started with the help of Britain in June 1916 at the Battle of Mecca, led by Sherif Hussein of Mecca, and ended with the Ottoman surrender of Damascus. Fakhri Pasha, the Ottoman commander of Medina, resisted for more than two and half years during the Siege of Medina.

Along the border of Italian Libya and British Egypt, the Senussi tribe, incited and armed by the Turks, waged a small-scale guerrilla war against Allied troops. The British were forced to dispatch 12,000 troops to deal with the Senussi. Their rebellion was finally crushed in mid-1916.