At its southern and eastern borders, however, another vigorous Middle Eastern religion rose up to challenge Christian unity and dominance. Born in the desert lands of the Arabian Peninsula, and like Christianity heir to the Judaic tradition of monotheism, Islam expanded across the Middle East in the sixth and seventh centuries, sweeping westward through North Africa and up into southern Spain. Between the eleventh and fourteenth centuries, Moslem domination of the Christian pilgrimage sites in the Middle East, particularly the city of Jerusalem, led to a series of Crusades in which European Christians attempted to win back “their” Holy Land. Finally, in 1453, Moslems captured the remnant of Constantinople, the capital of the Eastern Roman, or Byzantine, Empire founded by Constantine a thousand years earlier.