By 1000 A.D., Europe was coming out of the Dark Ages and into a period of expansionism. Agricultural advances increased the food supply, therein by increasing the population, which led to more wealth and commerce. In 1064 the eastern churches refused to submit to the Pope causing the Byzantine and Roman church to split. The last successful invasion of Britain was in 1066 when William of Normandy defeated King Harold at the Battle of Hastings.
While Europe was recovering and expanding, events in the Muslim world were just starting to heat up. The Turks invaded Persia and smashed the Byzantine army at the Battle of Manzikert in 1701. During this invasion there was a small but constant population of pilgrims traveling to and from the Holy Land. The Turks invasion severely disrupted this travel.
The Crusades were holy wars sponsored by the papacy for the recovery of the Holy Land from the Muslims from the late eleventh to the late thirteenth centuries. In 1095 Pope Urban II called for what would be known as the first Crusade. The stated objective was to drive the Turks out of Anatolia; however the hidden objective was to bring the Roman and Byzantine Churches back together as one. It took about a year to gather Crusaders. The first Crusade lasted two years and achieved its objectives in the Holy Land. The Crusaders slaughtered the Turks and halted their expansion. Then they marched into Palestine and captured Jerusalem and massacred the city.
The Crusaders had allied themselves with many unstable forces in the region. And when they assisted a Muslim ruler in attacking Damascus, there payback was that ruler launching a jihad against them. This event caused the second Crusade in 1147. This Crusade was not at all successful. The two land armies were destroyed, while the jihad gained strength.