See biographies by I. L. Plunket (1915) and W. T. Walsh (1987); W. H. Prescott, History of the Reign of Ferdinand and Isabella the Catholic (3 vol., 1838; abr. ed. 1962); J. H. Mariéjol, The Spain of Ferdinand and Isabella (1892, tr. 1961); R. B. Merriman, The Rise of the Spanish Empire, Vol. II (1918, repr. 1962); J. H. Elliott, Imperial Spain: 1469-1716 (1963).
Isabella's father's previous marriage to Agnes of Courtenay had been annulled, but he had succeeded in having his children from that marriage legitimised. Her half-brother Baldwin IV was recognised unanimously as king, as he was the only male available, but he suffered from leprosy (then incurable). The succession would therefore fall to either his full sister Sibylla, or, if her legitimisation were challenged, to his half-sister Isabella. Isabella's mother and the Ibelins had strong ambitions for her to succeed, although Baldwin IV's diplomacy regarding overseas marriages clearly indicates that he regarded Sibylla as his immediate heir.
In 1180, when Isabella was 8 (according to William of Tyre), she was betrothed to Humphrey IV of Toron, on the orders of her half-brother Baldwin IV, in payment of a debt of honour to Humphrey's grandfather Humphrey II who had been mortally wounded saving the king at Banias, and to remove her from the Ibelins' political orbit. They were married in 1183, when Humphrey was about 16 or 17 and Isabella 11. Reflecting the political aims of the marriage, it seems that Humphrey's mother, Stephanie of Milly, and his stepfather, Reynald of Chatillon, restricted Isabella's contact with her mother and stepfather thereafter.
On their wedding night the castle of Kerak was attacked by the forces of Saladin. According to the Old French Contination of William of Tyre (also known as the Chronicle of Ernoul), Humphrey's mother Stephanie sent a message to Saladin telling him of the recent wedding and reminding him of their shared history:
However, this may be rather fanciful: there is no record in Arabic sources of Saladin having spent any time as an enslaved prisoner at Kerak.
Baldwin IV died in spring 1185, shortly after ordering a formal public crown-wearing by Baldwin V at the Church of the Holy Sepulchre, at which Balian of Ibelin carried the child-king, to signify that Isabella's family accepted his claim. Raymond III of Tripoli was now regent once more. Baldwin V, never a healthy child, died in summer 1186. Isabella's supporters questioned Sibylla's legitimacy, due to the annulment of her parents' marriage, but this position was not universally subscribed to. Had Sibylla not been married to Guy, she would have succeeded with less contention. Now, that both Baldwin IV and Baldwin V were dead, it was easier to resurrect that legal grounds for the succession of Isabella, who at 14 was no longer a minor.
The provisions of Baldwin IV's settlement were ignored. Sibylla was crowned as queen regnant in 1186, and, ignoring demands that she have her marriage annulled, she crowned Guy herself. Isabella's supporters, led by her mother Maria and stepfather Balian, and Raymond of Tripoli, had gathered in Nablus. They expected Humphrey to assert her claim, but, when given the opportunity to do so, he submitted to Guy instead.
Her supporters, notably her mother Maria and Balian of Ibelin, realised that she needed a suitable king - who was not her current husband. This situation was not without precedent: Isabella's father had been forced to divorce his first wife in order to succeed to the throne, and Sibylla had been pressed - but had refused - to have Guy annulled. Humphrey of Toron, whom Isabella liked very much, having practically grown up with him, had no great desire to be king. He had let down her cause in 1186, and was still a staunch supporter of Guy. He was more of a diplomat than a warrior, and even the Itinerarium Peregrinorum , which was highly sympathetic to him, suggests he was slightly effeminate. In autumn 1190, Maria and Balian abducted Isabella from Humphrey, and forced her to consent to an annulment because she had been under-age at the time of her marriage, and had been coerced by her half-brother, Baldwin IV. They intended to marry her to the ambitious Conrad, who was Baldwin V's nearest male kinsman, and had already proved himself capable politically and militarily.
After much political pressure, and a challenge to a trial by combat by Guy III of Senlis (which he refused), Humphrey consented to an ecclesiastical annulment by Ubaldo Lanfranchi, Archbishop of Pisa, who was Papal legate, and Philip of Dreux, bishop of Beauvais, who was a second-cousin of Conrad. Philip married Conrad and Isabella on 24 November, despite objections that the marriage was canonically incestuous (Isabella's half-sister Sibylla having been married to Conrad's older brother). Some modern popular writers have suggested this was a grim fate for the young queen, to be married off to a "grizzled old warrior" (David Boyle, Blondel's Song (2005), p. 63) who had twice been married before. However, Conrad, then about 45, was an intelligent, well-educated, handsome man of great personal courage and vitality - so perhaps her situation was less bleak than some have implied. She compensated Humphrey by restoring to him his title to Toron, Chastel Neuf and related estates, which had been taken into the royal domain on their marriage, before returning to Tyre with her new husband.
By virtue of his marriage to Isabella, Conrad became de jure uxoris King of Jerusalem. However, for seventeen months there was an interregnum during which Guy of Lusignan, despite the death of Sibylla, continued his claim. Guy's chief supporter was Richard I of England, his family's overlord in Poitou, while Isabella and Conrad's was Philip II of France, the son of Conrad's cousin Louis VII. Eventually, after Philip's departure, Conrad's kingship was confirmed by election in April 1192.
The news was brought to the couple in Tyre by Count Henry II of Champagne, the nephew of both the Kings of England and of France, who then returned to Acre. Only a few days later, on 28 April, Isabella and her ladies were late for dinner through lingering at the hammam. Conrad called on the Bishop of Beauvais, hoping to dine with him, but finding the bishop had already dined, set off back to the palace. On the way, he was set upon in the street and stabbed by Hashshashin. He died of his wounds that same day. Isabella was already known to be carrying their first child - Maria of Montferrat, who later succeeded her mother as queen regnant (see the Old French Contination of William of Tyre, the Brevis Regni Hierosolymitani Historia in the Annals of Genoa, and the Muslim chronicler Imad ad-Din al-Isfahani).
Two days later, Henry of Champagne returned to Tyre as the envoy of his uncle King Richard - and immediately betrothed himself to Isabella. According to some chroniclers, the people of Tyre were reportedly so taken by Henry's youth and handsomeness that they shouted that he should marry their princess, and Isabella immediately fell in love with him - but this is to put a romantic gloss on what was primarily a political pairing. Richard had supported Guy, and was suspected of involvement in Conrad's death. A marriage between his nephew and Isabella was more advantageous to him than reinstating Humphrey, despite the latter's objections. Henry and Isabella were married only eight days after Conrad's murder. Imad ad-Din al-Isfahani, who was present for the wedding, wrote:
Henry died in 1197 when a balcony or window-trellis gave way and he fell out of a window. He and Isabella had two daughters, Alice (born 1196) and Philippa (born 1197). After his death, Isabella was married for a fourth time to Amalric II of Jerusalem (also Amalric I of Cyprus), brother of Guy of Lusignan. They had two daughters, Sybilla (born 1198) and Melisende (born 1200), and one son, Amalric (1201–1205). King Amalric died in 1205 of food-poisoning caused by white mullet, shortly before his wife, and shortly after their infant son.
On her death in 1205, Isabella was succeeded as Queen of Jerusalem by her eldest daughter Maria of Montferrat.
The legality of Isabella's divorce from Humphrey was challenged in 1213, during the dispute over the succession to Champagne between her daughters Alice and Philippa and Henry's nephew Theobald IV. However, its validity seems to have been upheld: no challenge was made to the legitimacy of Maria and her descendants to succeed to the throne of Jerusalem, and in Champagne, Theobald bought off his cousins Alice and Philippa.
She is the title character of Alan Gordon's mystery novel, The Widow of Jerusalem (2003), which paints a more sympathetic portrait of her marriage to Conrad. She is introduced as a spoilt, vain young woman, but she matures in the course of the story. Only when it is too late does she realise that her husband loves her. His murder, and the later death of Henry, are investigated by the hero, the fool Theophilos (Feste).