A second factor in Somali history from the fifteenth century onward, in addition to southward migration, is the emergence of centralized state systems.

The most important of these in medieval times is Adal, whose influence at the height of its power and prosperity in the sixteenth century extends from Zeila, the capital, through the fertile valleys of the Jijiga and the Harer plateau to the Ethiopian highlands.

Adal's fame derives 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 Persian empire and much of Byzantium soon after the birth of Islam, there would be no jihad against Christian Ethiopia for centuries.

The forbidding Ethiopian terrain of deep gorges, sharp escarpments, and perpendicular massifs that rise more than fort-five hundred meters also discourages the Muslims from attempting a campaign of conquest against so inaccessible a kingdom.

The Somalis' connection with the Prophet Muhammad may be partly explained by the history of commercial and intellectual contact between the inhabitants of the Arabian and Somali coasts.

Early in the Prophet's ministry, a band of persecuted Muslims had fled, with the Prophet's encouragement, across the Red Sea into the Horn of Africa, where the Muslims were afforded protection by the Ethiopian negus, or king.

Thus, Islam may have been introduced into the Horn of Africa well before the faith took root in its Arabian native soil.

The large-scale conversion of the Somalis had to await the arrival in the eleventh, twelfth, and thirteenth centuries of Muslim patriarchs, in particular, the renowned Shaykh Daarood Jabarti and Shaykh Isahaaq, or Isaaq.

Daarood married Doombira Dir, the daughter of a local patriarch.

Their issue gave rise to the confederacy that forms the largest clan-family  in Somalia, the Daarood.

For his part, Shaykh Isaaq founds the numerous Isaaq clan-family in northern Somalia.

Along with the clan system of lineages, the Arabian shaykhs probably introduced into Somalia the patriarchal ethos and patrilineal genealogy typical of Semitic societies, and gradually replaced the indigenous Somali social organization, which, like that of many other African societies, may have been matrilineal.

Islam's penetration of the Somali coast, along with the immigration of Arabian elements, inspired a second great population movement reversing the flow of migration from northward to southward.

This massive movement, which will ultimately takw the Somalis to the banks of the Tana River and to the fertile plains of Harer in Ethiopia, had commenced in the thirteenth century and continues to the nineteenth century.

At this point, European interlopers appear on the East African scene, ending Somali migration onto the East African plateau.

Muslim-Christian relations sour during the reign of the aggressive Negus Yeshaq (ruled 1414-29).

Forces of his rapidly expanding empire descend from Ethiopia's highlands to despoil Muslim settlements in the valley east of the ancient city of Harar.

Having branded the Muslims "enemies of the Lord," Yeshaq invades the Muslim kingdom of Ifat in 1415.

He crushes the armies of Ifat and puts to flight in the wastes along the Gulf of Tadjoura (in present- day Djibouti) Ifat's king Saa'd ad Din.

Yeshaq follows Sa'ad ad Din to the island off the coast of Zeila (which still bears his name), where the Muslim king is killed.

Yeshaq compels the Muslims to offer tribute, and also orders 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.

The Muslims have recovered sufficiently by the sixteenth century to break through from the east into Ethiopia's central highlands.

Led by the charismatic Imam Ahmad ibn Ibrahim al-Ghazi (1506-43), the Muslims pour into Ethiopia, using scorched-earth tactics that decimated the population of the country.

A Portuguese expedition led by Cristóvão da Gama, a son of Vasco da Gama who is looking for the Prester John of medieval European folklore—a Christian, African monarch of vast dominions—arrives from the sea and saves Ethiopia.

The joint Portuguese-Ethiopian force uses cannon to rout the Muslims, whose imam dies on the battlefield.

Berbera replaces Zeila as the northern hub of Islamic influence in the Horn of Africa.

By the middle of the sixteenth century, Saylac and Berbera have become dependencies of the sharifs of Mecca and in the seventeenth century pass to the Ottoman Turks, who exercise authority over them through locally recruited Somali governors.

The Ottoman Empire is a world power when Suleyman dies in 1566.

Most of the great cities of Islam—Mecca, Medina, Jerusalem, Damascus, Cairo, Tunis, and Baghdad— are under the sultan's crescent flag.

The Porte exercises direct control over Anatolia, the sub-Danubian Balkan provinces, Syria, Palestine, and Mesopotamia.

Egypt, Mecca, and the North African provinces are governed under special regulations, as are satellite domains in Arabia and the Caucasus, and among the Crimean Tartars.

In addition, the native rulers of Wallachia, Moldavia, Transylvania, and Ragusa (Dubrovnik) are vassals of the sultan.

Wahhabi fervor is also significant in the history of the present-day UAE.

The Al Qasimi tribes that have controlled the area since the eighteenth century adapt Wahhabi ideas and transfer the movement's religious enthusiasm to the piracy in which they had traditionally engaged.

Whereas Wahhabi thought opposes all that is not orthodox in Islam, it particularly opposes non-Muslim elements such as the increasing European presence in the Persian Gulf.

The increased European presence results in large part from commercial competition between Al Qasimi merchants and British merchants for the lucrative trade between India and the Persian Gulf in the early nineteenth century.

British merchants enlist the British navy to assist them by launching attacks on Al Qasimi strongholds in the present-day UAE as early as 1809.

The navy does not succeed in controlling the situation until 1819, in which year, the British send a fleet from India that destroys Ras al Khaymah, an Al Qasimi port at the eastern end of the gulf.

From Ras al Khaymah, the British fleet destroys Al Qasimi ships along both sides of the gulf.

The British have no desire to take over the desolate areas along the gulf; they only wish to control the trading cities.

The British decide to leave most tribal leaders in power and conclude a series of treaties with them.

As a result of these truces, the Arab side of the gulf comes to be known as the "Trucial Coast."

This area had previously been under the nominal control of the sultan in Oman, although the Trucial Coast tribes are not part of the Ibadi imamate.

The area has also been referred to as "Trucial Oman" to distinguish it from the part of Oman under the sultan that is not bound by treaty obligation.

The British in 1820 seem primarily interested in controlling the Al Qasimi, whose main centers are Ras al Khaymah, Ajman, and Sharjah, which were all small ports along the southeastern gulf coast.

The original treaties, however, also involve Dubai and Bahrain, which are entrepôts.

The inclusion of these ports bring two other extended families, the Bani Yas and the Al Khalifa, into the trucial system.

Wahhabi thought has a special impact on the history of Qatar.

Muhammad ibn Abd al Wahhab's ideas prove popular among many of the peninsula tribes, including the Al Thani, before the Al Khalifa attempt to take over the area from Bahrain at the beginning of the nineteenth century.

As a result, Wahhabi beliefs motivate Al Thani efforts to resist the attempt of the Al Khalifa, who reject Wahhabism, to gain control of the peninsula.

Wahhabism will distinguish Qatar religiously from its neighbors in the early twenty-first century.

Tribes from the interior have always raided settled communities along the coast, but the Wahhabi faith provides them with a justification for continuing these incursions to spread true Islam.

Accordingly, in the nineteenth century Wahhabi tribe under the leadership of the Al Saud move at various times against Kuwait, Bahrain, and Oman.

The Wahhabi faith creates internal dissension in Oman as well as an external menace because it proves popular with some of the Ibadi tribes in the Omani interior.

Arabia in general and the gulf in particular experience a turbulent time in the eighteenth and nineteenth centuries.

To the southeast, the Al Said of Oman are extending their influence northward, and from Iraq the Ottoman Turks are extending their influence southward.

From the east, both the Iranians and the British are becoming increasingly involved in Arab affairs.

The most significant development in the region, however, is the Wahhabi movement.

The name Wahhabi derives from Muhammad ibn Abd al Wahhab, who dies in 1792.

He had grown up in an oasis town in central Arabia where he studied Hanbali law, usually considered the strictest of Islamic legal schools, with his grandfather.

While still a young man, he had left home and continued his studies in Medina and then in Iraq and Iran.

When he returns from Iran to Arabia in the late 1730s, he attacks as idolatry many of the customs followed by tribes in the area who venerate rocks and trees.

He extend his criticism to practices of the Twelve Imam Shia, such as veneration of the tombs of holy men.

He focuses on the central Muslim principle that there is only one God and that this God does not share his divinity with anyone.

From this principle, his students begin to refer to themselves as muwahhidun (sing., muwahhid), or "unitarians."

Their detractors refer to them as "Wahhabis."

Muhammad ibn Abd al Wahhab considers himself a reformer and looks for a political figure to give his ideas a wider audience.

He finds this person in Muhammad ibn Saud, the emire of Ad Diriyah, a small town near Riyadh.

The two swear a traditional Muslim pledge in 1744 in which they promise to work together to establish a new state (which will later become present-day Saudi Arabia) based on Islamic principles.

The limited but successful military campaigns of Muhammad ibn Saud cause Arabs from all over the peninsula to feel the impact of Wahhabi ideas.

The Wahhabis become known for a fanaticism similar to that of the early Kharijites.

This fanaticism helps to intensify conflicts in the gulf.