Mention the word "crusade" to anyone and you'll engender visions of either wild-eyed religious fanatics charging off to kill the infidels, or esteemed holy warriors taking up the burden of a religious mission far greater than themselves. There's no single judgment that can be made about the Crusades or even crusading generally, but it is a subject which merits closer attention than it usually receives.
What is crusading, exactly? The term "Crusade" can be used generally to refer to any of the military operations launched during the middle ages by the Catholic Church and Catholic political leaders against non-Catholic powers or heretical movements. Most crusades, however, were directed at Muslim states in the Middle East, with the first starting in 1096 and the last in 1270. The term itself is derived from the Latin cruciata, which means "cross-marked," i.e. cruce signati, those who wear the insignia of scarlet crosses.
Today the term "crusade" has generally lost its military implications (in the West, at least) and has acquired more metaphorical meanings. Within religion the label "crusade" may be applied to any organized drive to convert people to a particular brand of Christianity or just to stoke the fires of devotion and faith. Outside of religion the label is applied to reform movements or zealous undertakings designed to make significant changes in structures of power, authority, or social relationships.
Understanding the Crusades requires understanding that, contrary to traditional stereotypes, they weren't simply an aggressive military campaign against Muslim lands, nor were they merely a defensive military campaign against Muslims on the Iberian peninsula and in the Mediterranean. The Crusades, all of them, were in the first place an attempt to impose orthodox Christianity via military force across a wide swath of territory, and second, the product of Christian contact with a militarily powerful, culturally self-confident, and economically expansionistic religious civilization.
The Crusades, but especially the "true" Crusades launched against Islam in the Middle East, are arguably the most important aspect of the Middle Ages. It was here that medieval warfare, art, politics, trade, religion, and ideals about chivalry all came together. Europe entered the crusading age as one type of society but left it transformed in vital ways which were not always immediately obvious, but which nevertheless contained the seeds of change which continue to impact European and world affairs today.
Furthermore, the Crusades also fundamentally altered the relationship between Christianity and Islam. Although they constituted a decisive military "win" for Islam, the image of barbaric Christian Crusaders continues to haunt Arab Muslim perspectives of Europe and Christianity, especially when combined with the more recent history of European colonialism in the Middle East. It is curious that an ostensibly Islamic military and political triumph could be transformed into a touchstone of Islamic defeat and despair.