The five-week siege began on 7 June 1099 and was carried out by the Christian forces of Western Europe mobilized by Pope Urban II after the Council of Clermont in 1095.

The city had been out of Christian control since the Muslim conquest of the Levant in 637 and had been held for a century first by the Seljuk Turks and later by the Egyptian Fatimids.

Godfrey of Bouillon, prominent among the Crusader leadership, was elected as the first ruler of Jerusalem At the Council of Piacenza in 1095, Pope Urban II received envoys from Byzantine Emperor Alexios I asking Western Christians for assistance in liberating large parts of the Eastern Roman Empire from the Seljuk Turks who had conquered large parts of the region since 1070.

[10] His appeal marked the beginning of the Crusades, a holy war for God, in which he guaranteed participants a place in heaven.

There was dissent among the princes over what to do next; Raymond of Toulouse, frustrated, left Antioch to capture the fortress at Maarrat al-Nu'man in the siege of Maarat.

Eventually, on January 13, 1099, Raymond began the march south, down the coast of the Mediterranean, followed by Robert of Normandy and Bohemond's nephew Tancred, who agreed to become his vassals.

To make the situation worse for the Crusaders, al-Dawla poisoned all the water wells in the surrounding area, and cut down all trees outside Jerusalem.

On June 7, 1099, the Crusaders reached the outer fortifications of Jerusalem, which had been recaptured from the Seljuk Turks by the Egyptian Fatimids only the year before.

[12] The Crusaders divided themselves into two large groups: Godfrey of Bouillon, Robert of Flanders and Tancred planned to besiege from north, while Raymond of Toulouse positioned his forces to the south.

After taking their positions, the Crusaders launched their first attack on June 13; the main problem was that they had no access to wood for the construction of siege equipment, because all the trees had been cut down.

This included: two massive wheel-mounted siege towers, a battering ram with an iron-clad head, and numerous scaling ladders and a series of portable wattle screens; now they were ready to attack[12] The Fatimids kept an eye on the preparation by the Franks and they set up their mangonels on the wall in the firing range once an assault began.

On 15 July the assault recommenced in the Northern front; Godfrey and his allies gained success and the Crusader Ludolf of Tournai was the first to mount the wall.

The Franks quickly gained a foothold on the wall, and as the city's defenses collapsed, waves of panic shook the Fatimids.

[13] On 15 July 1099, the crusaders made their way into the city through the tower of David and began massacring large numbers of the inhabitants, Muslims and Jews alike.

[15] According to historian of Middle Eastern History, Konrad Hirschler, the massacre during the sack of Jerusalem is a widely depicted event in popular culture, but its historical details remain difficult to verify and reconstruct.

Medieval chroniclers often exaggerated both troop numbers and casualty figures, making it challenging to determine a precise death toll.

[17][18] Atrocities committed against the inhabitants of cities taken by storm after a siege were normal in ancient[19] and medieval warfare by both Christians and Muslims.

[27] Tancred claimed the Temple quarter for himself and offered protection to some of the Muslims there, but he was unable to prevent their deaths at the hands of his fellow crusaders.

[28] Albert of Aachen, who personally was not present but wrote using independent interviews conducted with survivors back in Europe, wrote that even beyond the first round of slaughter that accompanied the fall of Jerusalem, there was another round, "On the third day after the victory judgement was pronounced by the leaders and everyone seized weapons and surged forth for a wretched massacre of all the crowd of gentiles which was still left...whom they had previously spared for the sake of money and human pity".

[37] The Gesta Francorum claims that on Wednesday, August 9, two and a half weeks after the siege, Peter the Hermit encouraged all the "Greek and Latin priests and clerics" to make a thanksgiving procession to the Church of the Holy Sepulchre.

In November 1100, when Fulcher of Chartres personally accompanied Baldwin on a visit to Jerusalem, they were greeted by both Greek and Syrian clerics and laity (Book II, 3), indicating an Eastern Christian presence in the city a year later.

On 22 July, Godfrey of Bouillon, who had played the most fundamental role in the city's conquest, was made Advocatus Sancti Sepulchri ("advocate" or "defender" of the Holy Sepulchre).

Jerusalem remained in Christian hands for almost a century until the crusaders were defeated by Saladin at the Battle of Hattin in 1187, and three months later, the last defenders were expelled from the city.