The Crusades were a series of religious wars between Christians and Muslims.  The eight big crusade expeditions occurred between 1096 and 1291, mostly in the Middle East.  

The crusades started as a fight for control over the Holy Land.  This is the land now called Israel.  The Holy Land is very important for the three major monotheistic religions: Islam, Judaism, and Christianity.  Muslims believe the Prophet Mohammed visited the Holy Land and rose to Heaven from Jerusalem.  Christians believe Jesus was born, crucified, and resurrected in the Holy Land.  The Dome of the Rock, the Wailing Wall, the Temple Mount, and the Mount of Olives are all located there.  So are many other important religious sites.

During the Caliphate of Umar in the 7th century, Muslim forces had taken control of Jerusalem, Nazareth, Bethlehem, and other important religious sites.  In the crusades, Christian forces tried to re-take control of the Holy Land.

There were many different crusades.  The biggest and most important happened between the 11th century and the 13th century.  During this time, there were 9 large crusades.  They are numbered 1 through 9.  

There were also many smaller crusades.  These continued through the 16th century, until the Renaissance and Reformation. Some crusades even happened within Europe (for example, in Germany, Austria, and Scandinavia). 

The word "crusade" is related to the word "cross", and means a Christian holy war.  Muslims sometimes use the Arabic word “jihad" to describe the crusades.

First Crusades 
In 1095 the ruler of the Byzantine Empire, Alexius I, called for help to defend his empire against the Seljuk Turks. Pope Urban II asked all Christians to join a war against the Turks. The Pope told Christians that fighting the war would repay God for their sins. He said that anyone who died on a crusade would go straight to heaven. 

The Christian soldiers who joined the crusade were called "crusaders". They marched to Jerusalem, attacking several cities on their way. In 1099 they won the battle for Jerusalem. 

As a result of the First Crusades, four crusader states were created. These were the County of Edessa, the Principality of Antioch, the County of Tripoli and the Kingdom of Jerusalem.

Second Crusade 
After about 50 years of peace, Bernard of Clairvaux called for a new crusade after the Turks attacked the town of Edessa. French and German armies marched to the Holy Land in 1147, but were defeated. However, on the way, the Crusaders helped the Portuguese capture Lisbon from Al-Andalus as part of the Reconquista.

Third Crusade 
In 1187, after Saladin recaptured Jerusalem, Pope Gregory VIII called for a new crusade. Three of Europe's kings led this Third Crusade: Philip II of France, Richard I of England and Frederick I, Holy Roman Emperor. 

The Third Crusade was partially successful. The Crusaders took back control of Arsuf and Jaffa, two important cities in the Holy Land. However, they did not have enough soldiers to try to recapture Jerusalem. Instead, they made a truce with Saladin that let Christians travel safely through Jerusalem. The Crusaders then re-established the Kingdom of Jerusalem in Acre.

In 1190, Frederick drowned in Cilicia. In 1192, Richard left the Holy Land. On his way home, his ship was wrecked, leading him to Austria. There, he was captured and ransomed by his enemy, Duke Leopold.

Fourth Crusade 
Pope Innocent III started the Fourth Crusade in 1202. His plan was to attack the Holy Land through Egypt. To do this, he needed a fleet of ships. The Venetians agreed to build ships and train sailors for the crusade. The Venetians then changed the goal of the crusade, and went to the Christian city of Constantinople, where they tried to place a Byzantine exile on the throne. After a series of misunderstandings and outbreaks of violence, the city was sacked in 1204.

Albigensian Crusade 

The Albigensian Crusade was started in 1209 to eliminate the Cathars of southern Europe

Children's Crusade 
The Children's Crusade is a crusade of 1212. An outburst of the old popular enthusiasm led a gathering of children in France and Germany. A boy, from either France or Germany, said that Jesus had visited him, and told him to peacefully convert Muslims to Christianity. Following this vision, many children formed bands, and marched to Italy. There, they were put onto ships which either capsized in a storm, or which went to Morocco. Most of the children either starved to death or were sold into slavery.

More recent research 
In the first movement, Nicholas, a shepherd from Germany, led a group across the Alps and into Italy in the early spring of 1212. About 7,000 arrived in Genoa in late August. However, their plans did not bear fruit when the waters failed to part as promised and the band broke up. Some left for home, others may have gone to Rome, while still others may have travelled down the Rhône to Marseille where they were probably sold into slavery. Few returned home and none reached the Holy Land.

The second movement was led by a "shepherd boy" named Stephen de Cloyes near the village of Châteaudun. In June of that year, the boy said that he had a letter for the king of France from Jesus. He could gather a crowd of over 30,000 and went to Saint-Denis. There he was seen to work miracles. On the orders of Philip II, on the advice of the University of Paris, the crowd was sent home, and most of them went. None of the contemporary sources mentions plans of the crowd to go to Jerusalem.

Later chroniclers elaborated on these events. Recent research suggests those taking part were not children, at least not the very young. In the early 1200s, bands of wandering poor started cropping up throughout Europe. These were people displaced by economic changes at the time which forced many poor peasants in northern France and Germany to sell their land. These bands were referred to as pueri (Latin for "boys") in a condescending manner, in much the same way that people from rural areas in the United States are called "country boys."

In 1212, a young French puer named Stephen and a German puer named Nicholas separately began claiming that they had each had similar visions of Jesus. This resulted in these bands of roving poor being united into a religious protest movement which transformed this forced wandering into a religious journey. The pueri marched, following the Cross. They associated themselves with Jesus's biblical journey. This, however, was not a prelude to a holy war.

At that time, chronicles were mostly kept by the Catholic Church. They were written in Latin.

Thirty years later, chroniclers read the accounts of these processions and translated pueri as "children" without understanding the usage. So, the Children's Crusade was born. The resulting story illustrates how ingrained the concept of Crusading was in the people of that time— the chroniclers assumed that the pueri must have been Crusaders. In their innocence, they returned to the foundations of crusading characteristic of Peter the Hermit, and met the same sort of tragic fate.

According to Matthew Paris, one of the leaders of the Children's Crusade became "Le Maître de Hongrie," the leader of the Shepherds' Crusade in 1251.

Fifth Crusade 
During 1213, Pope Gregory IX pushed Frederick II into leading the Fifth Crusade.
The Church tried another crusade to retake the Holy Land. A crusading force from Hungary, Austria, and Bavaria captured Damietta, a city in Egypt, in 1219. The crusaders had to surrender, due to losing the battle for Cairo.

Sixth Crusade 
In 1228, Emperor Frederick II set sail from Brindisi for Syria. He did this after the Pope excommunicated him. By talking to the Turks he had success, and Jerusalem, Nazareth, and Bethlehem was given to the Crusaders for ten years without fighting. This was the first major crusade not initiated by the Papacy, a trend that was to continue for the rest of the century. This crusade only lasted for a year, from 1228 to 1229.

Seventh Crusade 
The Templars argued with Egypt in 1243. In 1244, Egypt attacked Jerusalem. Louis IX of France started a crusade against Egypt from 1248 to 1254. It was a failure and Louis spent much of the crusade living in the city of Acre. In the midst of this crusade was the first Shepherds' Crusade in 1251.

Eighth Crusade 
The Eighth Crusade was organized by Louis IX of France in 1270, to help the Crusader states in Syria. However, the crusade got as far as Tunis, where Louis died a month later.

Ninth Crusade 
Before he was the king, Edward I of England started a crusade in 1271. He retired the following year after a truce.

The end of the Crusades 
In time, the people went on Crusades for other purposes. The Crusades ended two centuries after they had begun, achieving mixed results. The crusades ended with the Mamluk Fall of Acre in 1291. (the link is not yet started).

References

Books and texts 

 Frederick Russell, "Children's Crusade", Dictionary of the Middle Ages, 1989, 
 Peter Raedts, "The Children's Crusade of 1212", Journal of Medieval History, 3 (1977), summary of the sources, issues and literature.
 Chronica Regiae Coloniensis, a (supposedly) contemporary source for the Children's crusade. From the Internet Medieval Sourcebook.
 The Children's Crusade: Fact or fable? , from The Straight Dope.

Other websites 
 Crusade Siege  Siege of a Mohammedan fortification during the crusades
 Crusades Citizendium

Crusades