At first the kingdom was little more than a loose collection of towns and cities captured during the crusade, but at its height in the mid-12th century, the kingdom encompassed roughly the territory of modern-day Israel, Palestine and the southern parts of Lebanon. From the Mediterranean Sea, the kingdom extended in a thin strip of land from Beirut in the north to the Sinai Desert in the south; into modern Jordan and Syria in the east, and towards Fatimid Egypt in the west. Three other crusader states founded during and after the First Crusade were located further north: the County of Edessa (1097–1144), the Principality of Antioch (1098–1268), and the County of Tripoli (1109–1289). While all three were independent, they were closely tied to Jerusalem. Beyond these to the north and west lay the states of Armenian Cilicia and the Byzantine Empire, with which Jerusalem had a close relationship in the twelfth century. Further east, various Muslim emirates were located which were ultimately allied with the Abbasid caliph in Baghdad.
The fragmentation of the Muslim east allowed for the initial success of the crusade, but as the 12th century progressed, the kingdom's Muslim neighbours were united by Nur ad-Din Zangi and Saladin, who vigorously began to recapture lost territory. Jerusalem itself fell to Saladin in 1187, and in the 13th century the kingdom was reduced to a few cities along the Mediterranean coast. In this period, the kingdom was ruled by the Lusignan dynasty of the Kingdom of Cyprus, another crusader state founded during the Third Crusade. Dynastic ties also strengthened with Tripoli, Antioch, and Armenia. The kingdom was soon increasingly dominated by the Italian city-states of Venice and Genoa, as well as the imperial ambitions of the Holy Roman Emperors. Emperor Frederick II (reigned 1220-1250) claimed the kingdom by marriage, but his presence sparked a civil war (1228-1243) among the kingdom's nobility. The kingdom became little more than a pawn in the politics and warfare of the Ayyubid and Mamluk dynasties in Egypt, as well as the Khwarezmian and Mongol invaders. As a relatively minor kingdom, it received little financial or military support from Europe; despite numerous small expeditions, Europeans generally proved unwilling to undertake an expensive journey to the east for an apparently losing cause. The Mamluk sultans Baibars (reigned 1260-1277) and al-Ashraf Khalil (reigned 1290-1293) eventually reconquered all the remaining crusader strongholds, culminating in the destruction of Acre in 1291.