First Crusade participant Raymond IV, Count of Toulouse
The First Crusade (1096–1099) was the first of a series of religious wars, or Crusades, initiated, supported and at times directed by the Latin Church in the medieval period. The objective was the recovery of the Holy Land from Islamic rule. While Jerusalem had been under Muslim rule for hundreds of years, by the 11th century the Seljuk takeover of the region threatened local Christian populations, pilgrimages from the West, and the Byzantine Empire itself. The earliest initiative for the First Crusade began in 1095 when Byzantine emperor Alexios I Komnenos requested military support from the Council of Piacenza in the empire's conflict with the Seljuk-led Turks. This was followed later in the year by the Council of Clermont, during which Pope Urban II supported the Byzantine request for military assistance and also urged faithful Christians to undertake an armed pilgrimage to Jerusalem.
This call was met with an enthusiastic popular response across all social classes in western Europe. Mobs of predominantly poor Christians numbering in the thousands, led by Peter the Hermit, a French priest, were the first to respond. What has become known as the People's Crusade passed through Germany and indulged in wide-ranging anti-Jewish activities, including the Rhineland massacres. On leaving Byzantine-controlled territory in Anatolia, they were annihilated in a Turkish ambush led by the Seljuk Kilij Arslan at the Battle of Civetot in October 1096.
In what has become known as the Princes' Crusade, members of the high nobility and their followers embarked in late-summer 1096 and arrived at Constantinople between November and April the following year. This was a large feudal host led by notable Western European princes: southern French forces under Raymond IV of Toulouse and Adhemar of Le Puy; men from Upper and Lower Lorraine led by Godfrey of Bouillon and his brother Baldwin of Boulogne; Italo-Norman forces led by Bohemond of Taranto and his nephew Tancred; as well as various contingents consisting of northern French and Flemish forces under Robert Curthose (Robert II of Normandy), Stephen of Blois, Hugh of Vermandois, and Robert II of Flanders. In total and including non-combatants, the forces are estimated to have numbered as many as 100,000.
The crusader forces gradually arrived in Anatolia. With Kilij Arslan absent, a Frankish attack and Byzantine naval assault during the Siege of Nicaea in June 1097 resulted in an initial crusader victory. In July, the crusaders won the Battle of Dorylaeum, fighting Turkish lightly-armoured mounted archers. After a difficult march through Anatolia, the crusaders began the Siege of Antioch, capturing the city in June 1098. Jerusalem, under the Fatimids,  was reached in June 1099 and the Siege of Jerusalem resulted in the city being taken by assault from 7 June to 15 July 1099, during which its residents were ruthlessly massacred. A Fatimid counterattack was repulsed later that year at the Battle of Ascalon, ending the First Crusade. Afterwards the majority of the crusaders returned home.
Four Crusader states were established in the Holy Land: the Kingdom of Jerusalem under Godfrey of Bouillon but not using the title king, the County of Edessa, the Principality of Antioch, and the County of Tripoli. The Crusader presence remained in the region in some form until the loss of the last major Crusader stronghold in the Siege of Acre in 1291. After this loss of all Crusader territory in the Levant, there were no further substantive attempts to recover the Holy Land.The road to Constantinople
The armies travelled to Constantinople by various routes, with Godfrey taking the land route through the Balkans. Raymond of Toulouse led the Provençals down the coast of Illyria, and then due east to Constantinople. Bohemund and Tancred led their Normans by sea to Durazzo, and thence by land to Constantinople. The armies arrived in Constantinople with little food and expected provisions and help from Alexios. Alexios was understandably suspicious after his experiences with the People's Crusade, and also because the knights included his old Norman enemy, Bohemond, who had invaded Byzantine territory on numerous occasions with his father and may have even attempted to organize an attack on Constantinople while encamped outside the city. This time, Alexios was more prepared for the crusaders and there were fewer incidents of violence along the way.
The crusaders may have expected Alexios to become their leader, but he had no interest in joining them, and was mainly concerned with transporting them into Asia Minor as quickly as possible. In return for food and supplies, Alexios requested the leaders to swear fealty to him and promise to return to the Byzantine Empire any land recovered from the Turks. Godfrey was the first to take the oath, and almost all the other leaders followed him, although they did so only after warfare had almost broken out in the city between the citizens and the crusaders, who were eager to pillage for supplies. Raymond alone avoided swearing the oath, instead pledging that he would simply cause no harm to the empire. Before ensuring that the various armies were shuttled across the Bosporus, Alexios advised the leaders on how best to deal with the Seljuk armies that they would soon encounter.