The population of Egypt is concentrated chiefly along the fertile valley of the River Nile, the rest of the country being largely desert. Egypt’s history spans 5,000 years: the ancient kingdoms of Upper and Lower Egypt were ruled successively by thirty-one dynasties, which may be divided into the Old Kingdom, the Middle Kingdom, and the New Kingdom. Egypt was a centre of Hellenistic culture and then a Roman province before coming under Islamic rule and then becoming part of the Ottoman Empire. Modern Egypt became independent in 1922. From 1958 to 1961 Egypt was united with Syria as the United Arab Republic, a title it retained until 1971. Wars with Israel were fought in 1967 (the Six Day War) and 1973 (the Yom Kippur or October War); the countries signed a peace treaty in 1979. In 2011 a series of popular demonstrations led to President Hosni Mubarak leaving office