The Crusades were a series of military or para-military expeditions despatched by the religious and secular leaders of Christian Europe. Their target was the Islamic Middle East – in particular the regions of Palestine and Syria, where they hoped to rid the birthplace of Christianity of what they perceived to be Muslim tyranny. It was not merely piety which motivated many of the Europeans who embarked upon these ventures. Some sought adventure, while others were driven by greed for both riches and power in the East. Even the papacy may have had motivations of a less than spiritual character. Some view the Crusades as a method for the fulfilment of papal desire to extend its authority. The Crusades have even been seen as a way to limit fighting and feuding among the nobles of Northern and Western Europe. Animosities could now be vented against unbelievers. Many Muslims believed the Crusades to be a continuation of the process of Christian expansion, which had already met with success in southern Italy, Sicily, and parts of Spain.

Despite the complexity of motivation, the Crusades were initially triggered by the movement of Seljuk Turks into Asia Minor and the Holy Land. The Seljuks were less accommodating than the Arabs, who had wrested Palestine from the Byzantines in the 7th century. Christian pilgrimages to the holy sites in Palestine were increasingly restricted, and soon became dangerous propositions altogether. At the same time, the Byzantine Emperor Alexius is reputed to have sent a plea for assistance from the West against the Seljuks, who now posed a serious threat to the security of Constantinople. Pope Urban II saw this as an opportunity to both restore Christian control over the Holy Land and effect a reconciliation with the Eastern Christian Church. The potential for increasing the power of the Western Church seemed great indeed. There were eight official Crusades between 1096 and 1291.

The First Crusade (1096-1099)
This was the most successful of the Crusades. The Crusaders achieved their goal with the capture of Jerusalem and the subsequent establishment of a series of Christian states in Palestine and Syria, known collectively as Outremer, the "land across the sea."

The Second Crusade (1147-1149)
It did not take long for the Muslims to begin regaining some of the territory lost to the Crusaders. When the Christian county of Edessa fell to Islamic forces in 1144, a second Crusade was proclaimed. The Christians failed to recover Edessa and instead laid siege to the city of Damascus, which successfully held out against them. This served to heighten Muslim confidence.

The Third Crusade (1189-1192)
During the 1170's and 1180's, Egypt rose as a formidable power which managed to effect a strong Muslim coalition against the Crusader states. Under the charismatic and skilful general Saladin, Islamic forces were able to reconquer much of the territory that they had lost – including Jerusalem. A third Crusade was proclaimed in Europe to reverse this state of affairs. It was led by Holy Roman Emperor Frederick Barbarossa (who died en route), King Philip Augustus of France and King Richard I (the Lion-Heart) of England. After successfully retaking the coastal town of Acre in 1191, Richard and Philip quarrelled and Philip returned to France. As such, Richard would be the only Christian leader to distinguish himself in this Crusade, and he and Saladin proved to be worthy opponents. Richard, however, never managed to achieve his goal of reconquering Jerusalem.

The Fourth Crusade (1201-1204)
This Crusade was a fiasco that resulted in Western Christians attacking Eastern Christians. The Franks and Venetians, instead of proceeding to the Holy Land as originally planned, assaulted and captured Constantinople. They then set up a Latin regime in Byzantine territory that lasted until 1261.

The Fifth Crusade (1217-1221)
The Fifth Crusade was led by a papal legate and managed to successfully attack the Egyptian city of Damietta in 1218. A subsequent advance on Cairo met with defeat and the Crusaders retreated, giving up their gains, in 1221.

The Sixth Crusade (1228-1229)
Holy Roman Emperor Frederick II led this unusual Crusade which actually managed to regain Jerusalem through negotiation with the Sultan of Egypt in 1229. The Muslims did not allow this ignominy to last for very long.

The Seventh Crusade (1248-1251)
Louis IX (Saint Louis) of France led this Crusade, which like so many previous attempts, ultimately met with failure. Although he enjoyed an initial victory in his attack on Egypt, he was eventually captured by the Muslims and lost much of his army. He was not in captivity for long and managed to make a second attempt with the Eighth Crusade.

The Eighth Crusade (1270)
This was an aborted venture that ended with Louis' death from illness in Tunisia.

Although there would be numerous further forays by Christian troops against the Islamic East, the Crusading spirit was on the wane by the end of the 13th century. In fact, many of the later Crusading actions were defensive in nature, as the rising Muslim tide steadily eradicated Christian power in the East. The Christian-held city of Antioch fell in 1268, and the town of Acre, the last of the Crusader strongholds, finally succumbed in 1291. In 1258, the Christians were temporarily hopeful that they might have found a powerful new ally against their Muslim foes. The Mongols had arrived in strength from Central Asia and in that year, managed to destroy the Islamic Abbasid Caliphate and occupy Baghdad. Attempts by the Christians to forge an alliance with these mighty newcomers were dashed when, in 1260, the Muslims inflicted the first serious defeat on the Mongol hordes at the battle of Ayn Jalut in Syria.

The Crusades had few enduring political results, except to perpetuate existing animosities between Muslims and Christians. There were other results, however, of a non-political nature. European Crusaders returned home with many new ideas and innovations gleaned from the East. Arab science, mathematics, and astronomy began flowing into Europe, and would continue to do so from North African ports and through Moorish Spain. The Arabs had developed different technologies such as the windmill, which the Crusaders adopted and took back to Europe. But what truly awed many of the Crusaders was the scale and strength of Eastern fortifications, particularly the great walled cities such as Constantinople, Nicaea, Antioch, and Jerusalem itself. This stimulated a revival of military architecture in Europe. Stone began to replace wood in the construction of strongholds. The central square tower (or keep) of a fortress or castle, was now surrounded by outer defences such as concentric walls topped with crenellations, or battlements. These walls, in turn, were often surrounded by ditches or hedging. Castle construction, which grew throughout the 12th century in Europe, came to full fruition in the 13th century. Defensive walls were also built around many European towns. In architectural terms alone, the Crusades helped change the face of Europe.