Islam
The Turks
The Turks were another of those fierce, nomadic peoples produced every few centuries in the steppes of Asia. They began to filter into Muslim Persia in the 9th century, mainly as mercenaries and adventurers. Success in these roles brought land and titles to some, and so the Turks began to settle within Islam, converting to the faith. During the 10th century, most of the Turkish tribes converted to Islam, so when they began migrating in the 11th century, it was as Muslims, specifically as Sunnite Muslims.
By 1040, the Turks were moving into Persia in large numbers, sweeping away every opponent. Their conquests are known today as the Seljuk Empire, named after the leader who first united the tribes. In 1060 they drove the Shi'ite ruler from Baghdad, propping up the failing Abassid caliph there. Now, though, the Caliph was under the thumb of a new power: the Turk who ruled in the name of the Caliph took the title of Sultan. From that point on, the Caliph in Baghdad was a religious leader only, and it was the Sultan who ruled.
The Turks aimed in two directions: Byzantium and Egypt. The former was Christian and so ripe for conquest and conversion, while the latter was heretical (Shi'ite) and so open to attack. Under Alp Arslan, the Turks invaded Armenia and Georgia. In their train came thousands upon thousands of Turkish nomads, following after their victorious warriors. These settled in such numbers in Anatolia that gradually Asia Minor became known as Turkey.
The key event in this process was the battle at Manzikert in 1071, when Alp Arslan defeated and killed the Byzantine Emperor Romanus Diogenes. The Turks were still busily settling in to this area when the First Crusaders arrived, twenty-five years later.
To the south, at first it appeared the Turks would be equally successful. After a terrible six-year famine (1067-1071) caused the Fatimids to lose much of their empire, including Sicily and Palestine, Egypt recovered her strength. She was led by Badr al-Jamali, an Armenian who took the title of Amir (Commander of the Troops). He recovered Palestine, including Jerusalem, so it the city was under the control of Arabs and Egyptians when it fell to the Latins in 1099.
The Turks were unable to respond. They had no tradition of centralized government and from the 1090s on, their lands were never united but were ruled by independent princes who took various titles and quarreled with one another incessantly.