In addition to southward migration, a second factor in Somali history from the fifteenth century onward was the emergence of centralized state systems. The most important of these in medieval times was Adal, whose influence at the height of its power and prosperity in the sixteenth century extended from Saylac, the capital, through the fertile valleys of the Jijiga and the Harer plateau to the Ethiopian highlands. Adal's fame derived not only from the prosperity and cosmopolitanism of its people, its architectural sophistication, graceful mosques, and high learning, but also from its conflicts with the expansionist Ethiopians. For hundreds of years before the fifteenth century, goodwill had existed between the dominant new civilization of Islam and the Christian neguses of Ethiopia. One tradition holds that Muhammad blessed Ethiopia and enjoined his disciples from ever conducting jihad (holy war) against the Christian kingdom in gratitude for the protection early Muslims had received from the Ethiopian negus. Whereas Muslim armies rapidly overran the more powerful empires of Persia and Byzantium soon after the birth of Islam, there was no jihad against Christian Ethiopia for centuries. The forbidding Ethiopian terrain of deep gorges, sharp escarpments, and perpendicular massifs that rise more than 4,500 meters also discouraged the Muslims from attempting a campaign of conquest against so inaccessible a kingdom.

Muslim-Christian relations soured during the reign of the aggressive Negus Yeshaq (ruled 1414-29). Forces of his rapidly expanding empire descended from the highlands to despoil Muslim settlements in the valley east of the ancient city of Harer. Having branded the Muslims "enemies of the Lord," Yeshaq invaded the Muslim Kingdom of Ifat in 1415. He crushed the armies of Ifat and put to flight in the wastes along the Gulf of Tadjoura (in present-day Djibouti) Ifat's king Saad ad Din. Yeshaq followed Saad ad Din to the island off the coast of Saylac (which still bears his name), where the Muslim king was killed. Yeshaq compelled the Muslims to offer tribute, and also ordered his singers to compose a gloating hymn of thanksgiving for his victory. In the hymn's lyrics, the word Somali appears for the first time in written record.

By the sixteenth century, the Muslims had recovered sufficiently to break through from the east into the central Ethiopian highlands. Led by the charismatic Imam Ahmad Guray (1506-43), the Muslims poured into Ethiopia, using scorched-earth tactics that decimated the population of the country. A Portuguese expedition led by Pedro da Gama, a son of Vasco da Gama who was looking for the Prester John of medieval European folklore--a Christian, African monarch of vast dominions--arrived from the sea and saved Ethiopia. The joint Portuguese-Ethiopian force used cannon to route the Muslims, whose imam died on the battlefield.