Before the Crusades, Europe and the Middle East had a long and complex history of interaction. The Byzantine Empire, centered in Constantinople, was once a powerful force in the region. However, by the 11th century, the Byzantines were weakened by internal strife and territorial losses to the Seljuk Turks. In response, Pope Urban II called for a holy war against the Muslims to retake Jerusalem and the Holy Land. This marked the beginning of the Crusades, a series of religious wars that spanned over two centuries and had a profound impact on the political, social, and cultural landscapes of Europe and the Middle East.