The Ottoman Empire was a multi-ethnic, multi-religious empire with a central government lead by the Sultan in Constantinople (now Istanbul, Turkey). It had been in existence for 600 years, and at its zenith occupied most of the Middle East, much of south eastern Europe, most of North Africa, and extended well into Asia. The Sultan also served as the Caliph, or religious leader, for Sunni Muslims.
As the Ottoman Empire entered WWI, it consisted of what is now present day Turkey, Syria, Lebanon, Israel (part of Palestine), Palestine, Jordan, Iraq, part of Saudi Arabia, Kuwait, Qatar, Bahrain, Cyprus, part of Yemen, Greece (part of Eastern Thrace and some Aegean islands), part of Bulgaria, Georgia (part of Batumi Oblast).