1095 – 1291
In March 1095, Byzantine Emperor Alexios I Komnenos sent his ambassador to call for help with defending his empire against the Muslim Seljuk Turks. Pope Urban II, at the Council of Clermont held in November of the same year, summoned the attending nobility and the people to wrestle the Holy Land from the hands of the Seljuk Turks.
This led to the First Crusade. Crusader armies marched on Jerusalem, sacking several cities on their way. In 1099 they took Jerusalem and massacred the population. As a result of the First Crusade, several small Crusader states were created, notably the Kingdom of Jerusalem.
There followed a further six major Crusades against Muslim territories in the east, and numerous minor ones as part of an intermittent 200-year struggle for control of the Holy Land that ended in failure. After the fall of Acre, the last Christian stronghold in the Holy Land, in 1291, Catholic Europe mounted no further coherent response in the east.