The Crusades were a series of military expeditions launched by Christian Europe in an attempt to reclaim territory from the Muslims. In addition to waging war on the Muslims, the Crusaders also fought Paganism and numerous Christian sects. These holy wars were sanctioned by Popes and many civic leaders, and they had far-reaching repercussions, some of which continue to be felt today, despite the fact that they ended in the 14th century.
The backstory behind the Crusades is incredibly complex, but they were essentially sparked by two things: Muslim conquest of Christian holy sites such as Jerusalem, and social unrest in Europe. The Muslim conquest was a thorn in the side of many church leaders, while social unrest provided a pool of potential soldiers to be exploited. Some historians have suggested that the focus of the First Crusade in 1096 was actually brought about by concerns that political infighting would tear Europe apart; by sending warriors off to claim new territory, the Pope could help to stabilize the political situation in Europe.
From the start, the message of the Crusades was mixed. Although they were in theory holy expeditions, they were not very religious in nature. Crusaders were granted indulgences which allowed them to behave however they pleased while on Crusade, and many took advantage of this to loot, rape, and murder indiscriminately as they swept into Spain, the Baltic region, and the Middle East. While some undoubtedly did have religious ideals, many more were simply on a quest for glory, treasure, and land.
Initially, the Crusades were successful, and some formerly Christian territories were reclaimed. Subsequent expeditions sent out reinforcements to continue to hold the land against rising Muslim empires. However, ultimately the Islamic world proved to be too powerful, and the Christian strongholds were overwhelmed, with the Crusaders being beaten back into Europe by understandably upset Muslim warriors who wanted to reclaim the lands they thought of as home.
In addition to external Crusades, there were also expeditions in Europe, with Popes using them as a tool to weed out rivals and heretics. Many people were glad to see the Crusades come to an end, since numerous Europeans died or disappeared during them, and ignominious events such as the Children's Crusade in 1212 did nothing to promote a positive image of these allegedly holy wars.
As a result of these wars, some parts of the Middle East continue to experience tension between Christians and Muslims. Despite the fact that Muslims and Christians once got along in regions like Spain, the Crusades drove a wedge between these religious faiths, and that wedge continues to be a political and social issue today.