Definition of a "Crusade":
The medieval "Crusade" was a holy war. For a conflict to be officially considered a Crusade, it had to be sanctioned by the pope and conducted against groups seen as enemies of Christendom.
Initially, only those expeditions to the Holy Land (Jerusalem and associated territory) were considered Crusades. More recently, historians have also recognized campaigns against heretics, pagans and Muslims in Europe as Crusades.
How the Crusades Began:
For centuries, Jerusalem had been governed by Muslims, but they tolerated Christian pilgrims because they helped the economy. Then, in the 1070s, Turks (who were also Muslim) conquered these holy lands and mistreated Christians before realizing how useful their good will (and money) could be. The Turks also threatened the Byzantine Empire. Emperor Alexius asked the pope for assistance, and Urban II, seeing a way to harness the violent energy of Christian knights, made a speech calling for them to take back Jerusalem. Thousands responded, resulting in the First Crusade.
When the Crusades Began and Ended:
Urban II made his speech calling for Crusade at the Council of Clermont in November, 1095. This is seen as the start of the Crusades. However, the reconquista of Spain, an important precursor to crusading activity, had been going on for centuries.
Traditionally, the fall of Acre in 1291 marks the end of the Crusades, but some historians extend them to 1798, when Napoleon expelled the Knights Hospitaller from Malta.
There were as many different reasons for crusading as there were crusaders, but the single most common reason was piety. To crusade was to go on pilgrimage, a holy journey of personal salvation. Whether that also meant giving up virtually everything and willingly facing death for God, bending to peer or family pressure, indulging blood lust without guilt, or seeking adventure or gold or personal glory depended entirely on who was doing the crusading.
Who Went on Crusade:
People from all walks of life, from peasants and laborers to kings and queens, answered the call. Women were encouraged to give money and stay out of the way, but some went on crusade anyway. When nobles crusaded, they often brought huge retinues, whose members may not necessarily have wanted to go along. At one time, scholars theorized that younger sons more frequently went crusading in search of estates of their own; however, crusading was an expensive business, and recent research indicates it was lords and elder sons who were more likely to crusade.
The Number of Crusades:
Historians have numbered eight expeditions to the Holy Land, though some lump the 7th and 8th together for a total of seven crusades. However, there was a steady stream of armies from Europe to the Holy Land, so it is nearly impossible to distinguish separate campaigns. In addition, some crusades have been named, including the Albigensian Crusade, the Baltic (or Northern) Crusades, the Children's Crusade, the People's Crusade, and the Reconquista.
Upon the success of the First Crusade, the Europeans set up a king of Jerusalem and established what is known as the Crusader States. Also called outremer (French for "across the sea"), the Kingdom of Jerusalem controlled Antioch and Edessa, and it was divided into two territories since these places were so far-flung.
When ambitious Venetian merchants convinced warriors of the Fourth Crusade to capture Constantinople in 1204, the resulting government was referred to as the Latin Empire, to distinguish it from the Greek, or Byzantine, empire they had claimed.
Two important military orders were established in the early 12th century: the Knights Hospitaller and the Knights Templar. Both were monastic orders whose members took vows of chastity and poverty, yet they were also militarily trained. Their primary purpose was to protect and aid pilgrims to the Holy Land. Both orders did very well financially, particularly the Templars, who were notoriously arrested and disbanded by Philip IV of France in 1307. The Hospitallers outlasted the Crusades and continue, in a much-altered form, to this day. Other orders were established later, including the Teutonic Knights.
Impact of the Crusades:
Some historians -- particularly Crusades scholars -- consider the Crusades the single most important series of events in the Middle Ages. The significant changes in the structure of European society that took place in the 12th and 13th centuries were long considered the direct result of Europe's participation in the Crusades. This view no longer holds as strongly as it once did. Historians have recognized many other contributing factors in this complex time.
Yet there is no doubt the Crusades contributed greatly to changes in Europe. The effort of raising armies and providing supplies for Crusaders stimulated the economy; trade benefited, as well, especially once the Crusader States were established. Interaction between the East and West affected European culture in areas of art and architecture, literature and education. And Urban's vision of directing the energies of warring knights outward succeeded in reducing war within Europe. Having a common foe and common objective, even for those who didn't participate in the Crusade, fostered a view of Christendom as a united entity.