The Crusades
In 1095 Pope Urban II called on western knights to help Byzantium against the Turks and free Jerusalem. In 1099 the First Crusade took the city in a bloody massacre and founded crusader kingdoms along the coast.
Saladin united Egypt and Syria and retook Jerusalem in 1187. Later crusades failed, and the Fourth Crusade sacked Christian Constantinople instead.
The last crusader city, Acre, fell to the Mamluks in 1291. The crusades left bitter memories, but they also opened trade between Italy and the East.