The Byzantine emperor Alexios I had sent envoys to the papacy seeking help against Seljuk advances in Anatolia. At the Council of Clermont in November 1095 Pope Urban II issued a call that far exceeded the request: an armed pilgrimage to 'recover' Jerusalem. The response surpassed what he had expected — peasants, nobles, knights, townspeople, and even independent 'People's Crusade' bands set out.

The First Crusade (1096–1099) was a military success: Jerusalem fell in 1099, and the massacres that followed left deep scars across the region's Christian, Muslim, and Jewish populations. Four Latin states were founded in the east (Edessa, Antioch, Tripoli, Jerusalem). Over the following centuries crusades were launched again and again — some against Muslim powers, others (notoriously the Fourth Crusade of 1204) against Byzantine Constantinople, others against pagan peoples in northern Europe.

More lasting than their direct military outcomes was the cultural and economic transmission they accelerated. Europe forged deep trade ties with the Islamic world — Venice, Genoa, and Pisa grew wealthy in this era. Greek, Arabic, and Hebrew texts began to flow into the Latin West through translation; medical, astronomical, and mathematical knowledge reached the continent. At the same time, the crusades left a legacy of tension between Christian and Muslim worlds whose memory has been repeatedly mobilised in modern times.