Baldwin III (1130-1162) was king of Jerusalem from 1143-1162.
He was the eldest son of Fulk of Jerusalem and Melisende. He became king in 1143, under the regency of his mother, which lasted till 1152. At the beginning of his reign the Byzantine Empire was attempting to impose its influence over the Principality of Antioch in Cilicia, and Zengi of Mosul had invaded the County of Edessa. His reign marks the beginning of the decline of the Kingdom of Jerusalem, although it was still prosperous at the time.
In 1144 Edessa was captured by Zengi. The fall of the county lead to the call for the Second Crusade in Europe, which arrived in 1147. Baldwin welcomed Conrad III and Louis VII in Jerusalem in 1148, and the three kings planned an attack on Damascus. As Damascus was an ally of Jerusalem against Zengi, this plan was very foolish; the siege was a complete failure, and afterwards Damascus fell under the control of Zengi's successor Nur ad-Din.
In 1149, after the failure of the crusade, Baldwin III appeared in Antioch as regent after the fall of Raymund, the husband of the princess Constance, daughter of Bohemund II of Antioch. He tried to renew the truce with Damascus, and was forced to cede the last remnant of the County of Edessa, Tel-bashir, to Byzantine emperor Manuel I Comnenus.
In 1152 Baldwin finally attempted to take full control of Jerusalem from his mother, who was still acting as co-ruler. Melisende was unwilling to give up her power, so Baldwin attempted to have himself re-crowned as the sole king. Although this plan was unfeasible, he wore his crown publicly in the Church of the Holy Sepulchre as a symbol of his sole power. In the political struggle that followed, Baldwin agreed to let his mother rule in Jerusalem and Nablus, while he retained Acre and Tyre for himself.
Baldwin was not satisfied with this agreement, and convinced the other nobles of Jerusalem to ally with him. Ater some fighting Baldwin regained Jerusalem, but was soon summoned back to Antioch and the County of Tripoli to become regent once more. After returning to Jerusalem he defeated the Seljuk Turks in battle, and besieged Ascalon in the spring of 1153. Unlike in 1099 when the crusaders were unable to take the city, Baldwin successfully captured it, securing the border with Egypt.
In 1156 Baldwin was forced to sign a treaty with Nur ad-Din. However, in the winter of 1157—1158 Baldwin captured Harim, a former territory of Antioch, and in 1158 he defeated Nur ad-Din himself. That year he also married Theodora, a relative of Emperor Manuel I, and in 1159 he met with Manuel in Antioch. The two became friends, with Manuel adopting western clothes and customs and participating in a tournament against Baldwin. Manuel personally attended to Baldwin when the king was thrown from his horse during the tournament. Later in 1159 Baldwin became regent of Antioch once more, after Raynald of Chatillon, the second husband of Constance, had been captured in battle. In 1162 Baldwin died and was succeeded by his brother Amalric I.
Baldwin III was the first of the kings of Jerusalem who was born in Palestine. His three predecessors had all been emigrants from the West. Also unlike his predecessors, Baldwin was not involved in the affairs of the church, focusing his attention solely on the kingdom. William of Tyre says that Baldwin spent his spare time reading history and was knowledgeable in the jus consuetudinarium of the kingdom, later collected by lawyers like John of Ibelin and Philip of Novara as "the assizes of Jerusalem". He was well respected by both his subjects and his enemy Nur ad-Din, who remarked when Baldwin died "the Franks have lost such a prince that the world has not now his like." Cinnamus and Ibn-al-athir also wrote about Baldwin from the Byzantine and Muslim point of view.