After the conquest of the "Holy Lands" by the forces of Islam from the Eastern Roman Empire (aka Byzantine Empire), the Emperors in Constantinople tried to arrange a situation where they could regain these valuable territories. In the late 1090's Emperor Alexios I, in desperation, appealed to Pope Urban II in Rome for Western assistance in the recapture of these lands. Evidently Emperor Alexois I thought the warriors the Pope would send would be under the Emperors control and would return the land to the empire-nothing could be further from the truth.
Pope Urban II saw the Emperor's letter as a rallying cry for all good Christian warriors to turn from warring among themselves and expend their energies in recapturing the Holy Land for the "true" religion, as well as for the greater glory of God, Pope Urban II and the Roman Catholic (rather than the Greek Orthodox Faith of the Byzantine Empire) Faith. The knights and other "Crusaders" would wear a cross to show their devotion, would be given forgiveness for any sins they had committed, be guaranteed a place in Heaven, and get to keep any loot gained in the war.
So the first Crusades were launched. Huge numbers (some sources say hundreds of thousands) of knights, retainers, men-at arms, camp followers, peasants, women and even children rallied to the call and marched on the Holy Land. Tens of thousands died on the various marches, many were robbed and those that could not protect themselves were even sold into slavery. Those that made it to battle saw everyone they met as a potential enemy, even other Christians who they often killed. Fighting was bloody and brutal with massacre's common, as so often happens when killing in "God's Name."
In 1099, the Crusaders breached the walls of Jerusalem, and massacred everyone in the city they could find-Muslims, Jews and Christians, it made no difference to these warriors when their blood was up.
Rather than give up their newly conquered territories to the Byzantine Empire, the Crusaders set up a weak collection of Feudal states that could not and would not, support each other in time of need.
Ruled through arrogance and greed the Crusader states were in constant conflict with their Muslim neighbors and each other.
The results were predictable, in less than 100 years, the forces of Islam had taken back most of the lands the early Crusaders had captured. So the next major Crusade, usually known as the Third Crusade, was launched to re-recapture the Holy Lands. Three powerful leaders answered the call-Phillip II of France, Fredrick I "Barbarossa" of the German "Holy Roman Empire" and Richard I of England.
Richard I was a true warrior king and was given the title "Couer de Leon" (Lion-Hearted) for his courage in battle. He was an enthusiastic participant in the Third Crusade, being instrumental in the capture (and massacre of the 5000 man garrison) of Acre, while Phillip II, blaming illness soon returned to France and Fredrick I drowned crossing the Saleph River in Turkey. For two years Richard I struggled to regain Jerusalem but he was fought to a draw by the Great Saladin, Sultan of Egypt.
Saladin was a powerful warrior, firm in his religious belief as well as a man of honor and reason. When it became obvious that neither he nor Richard would be totally victorious the two leaders formed a truce that produced a treaty ending the conflict, and allowed Christians free access, as pilgrims to the Holy Lands. Proclaiming a victory of sorts, Richard I returned to Europe and his renewed conflict with Phillip II over the lands of France. The Third Crusade was over. More Crusades would follow, each less and less successful until by the end of the 13th century control of the Holy Lands was where it had been before the wars began, in the hands of the forces of Islam. Here they would remain until the First World War when the British would take the land from the dieing Ottoman Empire.
While a failure in their religious goal of regaining permanent Christian control of the Holy Lands, the Crusades were critical in future developments of European and World History. The money made in renewed trade, looted treasures from the Eastern Cities (including Constantinople which was brutally sacked by Western Europeans in 1204 during the "4th Crusade" which was anything but a Holy War) enriched the coffers of the "Kingdoms, City States, Nobles and Merchants" of the West. This wealth and rediscovered knowledge from the Byzantines and Muslims would hasten the end of the Middle Ages and the coming of the rebirth of science and arts in Europe-the Renaissance.
Three great warrior "Religious" orders of knights and men-at-arms were formed, the Knights Templar, the Teutonic Knights and the Hospitallers, to protect Pilgrims, expand the faith and tax to support themselves. Each would have important effects on the growth in power, influence and wealth of Western Europe.
The riches of the Silk Road opened up an insatiable appetite in Europe for the goods of the East. As it became more and more expensive to purchase these from the Islamic and Ottoman controlled trade cities, new trade routes were researched through discoveries of unknown lands. First by the Portuguese going South to round Africa to find a profitable route to "India." And then by a little known Italian who decided to sail West in search of the "Riches of the East" in 1492, and the World changed for ever.