Beginning in the 7th century, Muslim rulers began expanding their territories into Christian Roman/Byzantine lands, conquering Egypt and the Levant, and gradually taking over all of North Africa, much of Southwest Asia, and most of the Iberian Peninsula. The Eastern Romans, or Byzantines, partially recovered lost territory on numerous occasions but gradually lost all but Anatolia and parts of Thrace and the Balkans. In the West, the Roman Catholic kingdoms of northern Iberia launched campaigns known as the Reconquista to reconquer the peninsula from the Arabized Berbers known as Moors (who called it al-Andalus). The conquered Iberian principalities are not customarily called Crusader states, except for the Kingdom of Valencia, despite fitting the criteria.
Malcolm Barber, a British scholar of medieval history, indicates that, in the Crusader state of the Kingdom of Jerusalem the Holy Sepulchre was added to in the 7th century and rebuilt in 1022, "after a previous collapse". "In 691–2 Caliph Abd al Malik had built a great dome over the rock here, a place sacred to all three great religions". 
In 1071, the Byzantine army was defeated by the Muslim Seljuk Turks at the Battle of Manzikert, resulting in the loss of most of Asia Minor. The situation was a serious threat to the future of the Eastern Orthodox Byzantine Empire. The Emperor sent a plea to the Pope in Rome to send military aid to restore the lost territories to Christian rule. The result was a series of western European military campaigns into the eastern Mediterranean, known as the Crusades. Unfortunately for the Byzantines, the crusaders had no allegiance to the Byzantine Emperor and established their own states in the conquered regions, including the heart of the Byzantine Empire.