The Kingdom of Jerusalem was a crusader state that lasted from AD 1099 to 1291.
In the time of Christ Jerusalem was under the control of the Roman Empire. This control continued until the 7th century. With the conversion of Constantine (early 4th century), Christianity became a legal religion and eventually the state religion. Emperor Constantine built churches and shrines in Jerusalem and Palestine to commemorate holy sites. Jerusalem became a destination for Christian pilgrimages.
In the early 7th century, the Roman Empire lost control of Jerusalem. The city passed to Persian and then Arab-Muslim control. Islam also considered Jerusalem to be a holy site and built shrines and mosques there to commemorate events in their history. The Dome of the Rock is the most noteworthy shrine, built directly on the temple mount. Under Arab rule, Jerusalem prospered, and tolerance was extended to Christians at first. However, this tolerance began to wane over time.
In the early 11th century, a ruler of the Fatimid Dynasty ordered the destruction of all churches in Jerusalem. This outraged Christians throughout Europe and led to the First Crusade (1095–1099), ordered by Roman Catholic Pope Urban II. A crusade was a military expedition with spiritual significance. The First Crusade started as an expedition to help the Byzantine Emperor repel the Turkish Muslim invaders, but soon the focus became the recapture and liberation of Jerusalem. In 1099, the Crusader army laid siege to and captured Jerusalem, slaughtering many of the Muslim and Jewish inhabitants.
The captured territory was organized into crusader states—small territories or outposts governed by Western Europeans. The Kingdom of Jerusalem was one of these crusader states. Christian settlers immigrated to repopulate and rebuild Jerusalem, and Christian pilgrimages to Jerusalem resumed. This arrangement lasted until 1187 when the city and several other crusader states were captured by the Kurdish Muslim Saladin; fortunately, his official policy was one of tolerance to all religions. Although Jerusalem itself was in Muslim hands, the Kingdom of Jerusalem still existed as the small city of Acre on the coast.
This Muslim domination led to the Third Crusade, which was an attempt to drive Muslims from the Holy Land and reassert Christian/European control. The Crusaders were unable to recapture the city but negotiated a treaty with Saladin to allow pilgrimages. Fighting between various Muslim groups was common, and the city changed hands numerous times. Although the city did pass into Christian hands briefly about 50 years later, Acre was the last remaining vestige of the Kingdom of Jerusalem. It fell to the Muslims in 1291.
Jerusalem and Palestine remained in Muslim control until the 20th century when the United Nations partitioned Palestine, creating a Jewish state—modern Israel—after World War II.