A traditional numbering scheme for the crusades totals nine during the 11th to 13th centuries. This division is arbitrary and excludes many important expeditions, among them those of the 14th, 15th, and 16th centuries. In reality, the crusades continued until the end of the 17th century, the crusade of Lepanto occurring in 1571, that of Hungary in 1664, and the crusade to Candia in 1669.[13] The Knights Hospitaller continued to crusade in the Mediterranean Sea around Malta until their defeat by Napoleon in 1798. There were frequent "minor" Crusades throughout this period, not only in Palestine but also in the Iberian Peninsula and central Europe, against Muslims and also Christian heretics and personal enemies of the Papacy or other powerful monarchs.

First Crusade 1095-1099

In March 1095 at the Council of Piacenza, ambassadors sent by Byzantine Emperor Alexius I called for help with defending his empire against the Seljuk Turks. Later that year, at the Council of Clermont, Pope Urban II called upon all Christians to join a war against the Turks, promising those who died in the endeavor would receive immediate remission of their sins.

Following abortive popular crusades in early 1096, the official crusader armies set off from France and Italy on the papally-ordained date of 15 August 1096. The armies journeyed eastward by land toward Constantinople, where they received a wary welcome from the Byzantine Emperor. Pledging to restore lost territories to the empire, the Crusaders were supplied and transported to Anatolia where they laid siege to Seljuk-occupied Nicea. The city fell on 19 June 1097. The Crusader armies fought further battles against the Turks, facing grave deprivation of both food and water in their summer crossing of Anatolia. The lengthy Siege of Antioch began in October 1097 and endured until June of 1098. The ruler of Antioch was not sure how the Christians living within his city would react, so he forced them to live outside the citadel. The siege only ended when one of the gates to the city was betrayed by an Armenian dissident. Once inside the city, as was standard military practice at the time, the Crusaders massacred the Muslim inhabitants, destroyed mosques and pillaged the city. Local Christians assassinated Yaghisiyan, former ruler of the city. However a large Muslim relief army under Kerbogha immediately besieged the victorious Crusaders within Antioch. Bohemund of Taranto led a successful break-out and defeat of Kerbogha's army on the 28th of June. The starving crusader army marched south, moving from town to town along the coast, finally reaching the walls of Jerusalem on 7 June 1099 with only a fraction of their original forces.

Siege of Jerusalem

The Jews and Muslims fought together to defend Jerusalem against the invading Franks. They were unsuccessful though and on 15 July 1099 the crusaders entered the city. They proceeded to massacre the remaining Jewish and Muslim civilians and pillaged or destroyed mosques and the city itself. One historian has written that the "isolation, alienation and fear" felt by the Franks so far from home helps to explain the atrocities they committed, including the cannibalism which was recorded after the Siege of Maarat in 1098. As a result of the First Crusade, several small Crusader states were created, notably the Kingdom of Jerusalem. In the Kingdom of Jerusalem at most 120,000 Franks (predominantly French-speaking Western Christians) ruled over 350,000 Muslims, Jews, and native Eastern Christians.

The Crusaders also tried to gain control of the city of Tyre, but were defeated by the Muslims. The people of Tyre asked Zahir al-Din Atabek, the leader of Damascus, for help defending their city from the Franks with the promise to surrender Tyre to him. When the Franks were defeated the people of Tyre did not surrender the city, but Zahir al-Din simply said “What I have done I have done only for the sake of God and the Muslims, nor out of desire for wealth and kingdom.”

After gaining control of Jerusalem the Crusaders created four Crusader states: the kingdom of Jerusalem, the County of Edessa, the Principality of Antioch and the County of Tripoli. Initially, Muslims did very little about the Crusader states due to internal conflicts. Eventually, the Muslims began to reunite under the leadership of Imad al-Din Zangi. He began by re-taking Edessa in 1144. It was the first city to fall to the Crusaders, and became the first to be recaptured by the Muslims. This led the Pope to call for a second Crusade.

Crusade of 1101

Following this crusade there was a second, less successful wave of crusaders, in which Turks led by Kilij Arslan defeated the Crusaders in three separate battles in a well-managed response to the First Crusade. This is known as the Crusade of 1101 and may be considered an adjunct of the First Crusade.

Norwegian Crusade 1107-1110

Sigurd I of Norway was the first European king who went on a crusade and his crusader armies defeated Muslims in Spain, the Baleares, and in Palestine where they joined the king of Jerusalem in the Siege of Sidon.