Why did the Christian Byzantines engage in the Crusades?

The Christian Byzantines engaged in the Crusades primarily to halt the spread of Islam, particularly in the context of the threats posed by Muslim forces to Christian territories in the eastern Mediterranean. During the time leading up to the Crusades, the Byzantine Empire faced significant challenges, including territorial losses and military pressure from advancing Muslim armies. The Byzantines sought assistance from the West not only to combat these immediate threats but also to secure the safety of their own realm and to protect the Christian faith.

The appeal for military aid culminated in Pope Urban II's call for the First Crusade in 1095, which was partly motivated by the desire to reclaim Jerusalem and other Christian holy sites from Muslim control. The notion of defending Christendom was a powerful unifying theme that resonated with many Christian leaders and laypeople in Europe, driving them to travel to the East in the name of religion and to protect their way of life against perceived encroachment.

While other factors such as territorial acquisition, trade promotion, and the quest for religious freedom played roles in the broader context of the Crusades, the immediate motivation for the Byzantine engagement was predominantly the defense against the spread of Islam, which was viewed as a direct threat to the Byzantine Christian Empire and its values.