In 1071 the Seljuks achieved a stunning victory over a Byzantine army at Manzikert in modern Turkey that led to Turkish occupation of most of Anatolia. At nearly the same time, they successfully captured Jerusalem from its Egyptian Muslim rulers. These two events shocked the Byzantines, the Papacy, and the Europeans. The result was the Crusades that carried on for the next 200 years.

The Seljuk Turks were worn down by the recurring wars with the Crusaders, even though they were successful ultimately in regaining control of Palestine. They were threatened simultaneously by the activities of the Assassins, a heretical sect of Islam. Internally, Islam entered a period of introspection because of the popularity of Sufi mysticism. During this period of exhaustion and weakness, they were attacked suddenly by the Mongols and collapsed. Baghdad fell to the invaders in 1258 and the Seljuk Empire disappeared.

Islamic peoples from Anatolia (modern Turkey in Asia Minor) were unified in the early 14th century under Sultan Osman I and took the name "Osmanli," or "Ottomans," in his honor. The Ottomans swore a jihad against the crumbling Byzantine Empire and took their campaign around Constantinople into the Balkans and Greece. In 1389 the Serbs were defeated. In 1396 a "crusader" army from Hungary was defeated. Ottoman successes were temporarily halted by the Mongols under Tamerlane, but he moved on with his army and the Ottomans recovered.

Sultan Muhammad II ("the Conqueror") at last captured Constantinople on May 29, 1453. The great walls of Constantinople were battered by 70 guns for eight weeks and then 15,000 Janissaries led the successful assault.