History is not always pleasant, but it is important to know. It forces us to take a look at ourselves, and learn from previous mistakes. What would you say was the main reason for the crusades? Personally I would say it was a lack of knowledge of what it entails to be a be a Christian. Perhaps many of these individuals were what I would call just culturally Christian..?

Matthew 5:39
But I tell you, Do not resist an evil person. If someone strikes you on the right cheek, turn to him the other also.

Romans 12:19
Do not take revenge, my friends, but leave room for God's wrath, for it is written: "It is mine to avenge; I will repay," says the Lord.

A brief description and outline of the Cause of the Crusades is as follows:

* The massacre of 3000 Christian Pilgrims in Jerusalem prompted the first crusade

* Religious Conviction of crusaders

* The Instinct to Fight

* The Preaching of Peter the Hermit

* The Threat of the Turks

* The Council of Clermont led by Pope Urban II - "It is the will of God"

Leaders of the First Crusade
The leaders of the First Crusade included some of the most distinguished representatives of European knighthood. Count Raymond of Toulouse headed a band of volunteers from Provence in southern France. Godfrey of Bouillon and his brother Baldwin commanded a force of French and Germans from the Rhinelands. Normandy sent Robert, William the Conqueror's eldest son. The Normans from Italy and Sicily were led by Bohemond, a son of Robert Guiscard, and his nephew Tancred.

The First Crusade - The People's Crusade
The months which followed the Council of Clermont were marked by an epidemic of religious excitement in western Europe. Popular preachers everywhere took up the cry "God wills it!" and urged their hearers to start for Jerusalem. A monk named Peter the Hermit aroused large parts of France with his passionate eloquence, as he rode from town to town, carrying a huge cross before him and preaching to vast crowds. Without waiting for the main body of nobles, which was to assemble at Constantinople in the summer of 1096 a horde of poor men, women, and children set out, unorganized and almost unarmed, on the road to the Holy Land. This was called the Peoples Crusade, it is also referred to as the Peasants Crusade. Dividing command of the mixed multitudes with a poor knight, called Walter the Penniless, and followed by a throng of about 80,000 persons, among whom were many women and children, Peter the Hermit set out for Constantinople leading the Peoples Crusade via an overland route through Germany and Hungary. Thousands of the Peoples Crusade fell in battle with the natives of the countries through which they marched, and thousands more perished miserably of hunger and exposure. The Peoples Crusade was badly organised - most of the people were unarmed and lacked the command and discipline of the military crusaders. The Byzantium emperor Alexius I sent his ragged allies as quickly as possible to Asia Minor, where most of them were slaughtered by the Turks. The daughter of Alexius, called Anna Comnena wrote a book about her father and the crusaders called the Alexiad which provides historical details about the first crusaders. Those crusaders who crossed the Bosphorus were surprised by the Turks, and almost all of the Peoples Crusade were slaughtered. Peter the Hermit did survive and eventually led the Crusaders in a procession around the walls of Jerusalem just before the city was taken.

The Main Body of the First Crusade
Meanwhile real armies were gathering in the West. Recruits came in greater numbers from France than from any other country, a circumstance which resulted in the crusaders being generally called "Franks" by their Moslem foes. They had no single commander, but each contingent set out for Constantinople by its own route and at its own time.

The First Crusade - The Siege of Antioch
Godfrey of Bouillon, Duke of Lorraine, and Tancred, "the mirror of knighthood," were among the most noted of the leaders of the different divisions of the army. The expedition numbered about 700,000 men, of whom fully 100,000 were mailed knights. The crusaders traversed Europe by different routes and reassembled at Constantinople. Crossing the Bosphorus, they first captured Nicaea, the Turkish capital, in Bithynia, and then set out across Asia Minor for Syria. Arriving at Antioch, the survivors captured that place, and then, after some delays, pushed on towards Jerusalem. The Siege of Antioch had lasted from October 1097 to June 1098.

The First Crusade - The City of Jerusalem
Reduced now to perhaps one-fourth of their original numbers, the crusaders advanced slowly to the city which formed the goal of all their efforts. When at length the Holy City burst upon their view, a perfect delirium of joy seized the crusaders. They embraced one another with tears of joy, and even embraced and kissed the ground on which they stood. As they passed on, they took off their shoes, and marched with uncovered head and bare feet, singing the words of the prophet: "Jerusalem, lift up thine eyes, and behold the liberator who comes to break thy chains." Before attacking it they marched barefoot in religious procession around the walls, with Peter the Hermit at their head. Then came the grand assault.

The First Crusade - The Capture of Jerusalem
The first assault made by the Christians upon the walls of the city was repulsed; but the second was successful, and the city was in the hands of the crusaders by July 1099. Godfrey of Bouillon and Tancred were among the first to mount the ramparts. Once inside the city, the crusaders massacred their enemies without mercy. A terrible slaughter of the infidels took place. For seven days the carnage went on, at the end of which time scarcely any of the Moslem faith were left alive. The Christians took possession of the houses and property of the infidels, each soldier having a right to that which he had first seized and placed his mark upon.

http://www.middle-ages.org.uk/the-first-crusade.htm