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 , the Czar abdicated in the course of the February Revolution, and the Russian Caucasus Army began to fall apart. The Arab Revolt, instigated by the Arab bureau of the British Foreign Office, started June 1916 with 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 before surrendering in January 1919. The Senussi tribe, along the border of Italian Libya and British Egypt, 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 oppose them in the Senussi Campaign. Their rebellion was finally crushed in mid-1916. Total Allied casualties on the Ottoman fronts amounted 650,000 men. Total Ottoman casualties were 725,000 .. For how many years was General Yudenich, the Russian commander?