Marquis of Montferrat (1192-1207), titular leader of the Fourth Crusade (1202-1204) and subsequently marquis of Thessalonica (1204-1207).
Born around 1150, the third son of William V “the Old,” marquis of Montferrat, and Judith of Austria, Boniface succeeded to the marquisate of Montferrat in northwestern Italy in 1192 when his elder brother Conrad was assassinated in the Holy Land. In June 1201 the leading barons of the Fourth Crusade, perhaps on the advice of King Philip II of France, offered the leadership of the enterprise to Boniface, who accepted. After visiting the king in Paris and the monks of Cîteaux, he traveled to Haguenau in Alsace, where he spent Christmas with his cousin, Philip of Swabia, king of Germany. There he spoke with young Alexios Angelos, the brother of Philip’s wife, Irene, and son of the deposed Byzantine emperor, Isaac II Angelos. Alexios had just arrived in the West and was seeking an army to help him obtain the throne by overthrowing his uncle, Emperor Alexios III Angelos. The plan was commended by Philip, although, given his own struggle for the German throne, he could offer no material support. Boniface promised to do all that he could to help. He later tried and failed to convince Pope Innocent III to allow the crusade to divert to Constantinople.
The crusade fleet left Venice in early October 1202 without Boniface, who had matters to attend to elsewhere. He also wanted to avoid the conquest of Zara (mod. Zadar, Croatia), which the crusaders had agreed to undertake in return for the suspension of their debt to the Venetians, and did not arrive at Zara until mid-December. A few weeks later, envoys from Philip of Swabia arrived, promising rich rewards for the crusaders if they would help the young Alexios to claim his throne in Constantinople. Boniface naturally supported the plan, as did most of the other barons, conscious of the crusade’s lack of funds. Alexios joined the crusaders on 25 April 1202 and remained under the marquis’s care thereafter. After the crusaders had successfully placed their claimant on the throne of Byzantium as Emperor Alexios IV (July 1203), Boniface became an important member of the imperial court; it was probably at this time that Alexios granted him the island of Crete. Boniface led an expeditionary force with the new emperor to capture Alexios III and extend control over Thrace in autumn 1203, but soon after, the marquis was edged out of court by a growing anti-Latin faction.
Given their experience with the Montferrat clan, the Greek citizens of Constantinople believed that the crusader attack on the city in April 1204 was an attempt to place Boniface on the throne of the Caesars. After the city’s fall, people on the streets would greet Latins with the Greek phrase Ayos vasileas marchio (holy emperor the marquis). Boniface certainly aspired to the position, and he looked the part. He had already occupied the Great Palace of Constantinople and married the widow of Isaac II, Margaret of Hungary. However, the crusaders elected Count Baldwin IX of Flanders instead, and a dispute between the two men soon broke out over possession of the city of Thessalonica, which Boniface insisted should be given to him at once. War was avoided only by the quick work of Doge Enrico Dandolo and Geoffrey of Villehardouin. Boniface took the city and, with the help of his wife and her son, Manuel, extended his holdings throughout Thessaly and into central Greece. From 1205 onward, Boniface, allied with the new emperor, Henry, waged war against the Bulgarian emperor, Kalojan. He was killed on 4 September 1207 in a Bulgarian ambush near Mosynopolis, leaving a young son, Demetrius, as heir to Thessalonica.