Beginning in the 7th century, following the foundation of the Islamic religion by Muhammad and through the 8th century Muslim Arabs under the Umayyad Caliphate captured Syria, Egypt, Iran, the Levant and North Africa from the Byzantine Empire, and Iberia from the Visigothic Kingdom.
In 750 a bloody coup brought an end to Umayyad rule, leading to the gradual fragmentation of the monolithic Islamic state and the relocation of the political and economic centre of the Islamic world to Iran and Iraq and away from Palestine.  By the run up to the crusades at end of the 11th century the age of Islamic territorial expansion was long gone. However, frontier conditions between the Christian and Muslim world remained across the Mediterranean Sea. From the 8th century, in what later became known as the Reconquista, the Christians were campaigning in Spain and Norman adventurers led by Roger de Hauteville, later King Roger I of Sicily, seized Sicily from the Muslims. The ‘Holy Land’ had been under Arab Muslim control for more than four centuries with fluctuating levels of tolerance, trade, and political relationships between the Muslims and the Christians. Catholic pilgrims had access to sacred sites and Christian residents in Muslim territories were given Dhimmi status, legal rights, and legal protection. Indigenous Christians were allowed to maintain churches, and marriages between faiths were not uncommon. 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". 
The Orthodox Christian Byzantine Empire of Constantinople reached a zenith in early 11th century with frontiers stretching East to Iran while in the West controlling Bulgaria and much of southern Italy. However, from this point the arrival of new enemies on all frontiers placed intolerable strains on the resources of the Emipire and the neighbouring Arab Muslim regimes. This made the Byzantines susceptible to the opportunity presented by western military aid from the Papacy for specific campaigns. 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.