The Latin kingdom of Thessalonika (1204-1224)
After failing to be elected Latin emperor of Constantinople, the Lombard leader of the Fourth Crusade, Boniface of Monferrat (1204-1207) claimed the kingdom of Thessalonika. In August 1204, he signed a treaty of alliance with the Venetians, through which he became vassal of Venice and secured at the same time the support of the Serenissima in his confrontation with the Latin emperor. After a short, albeit violent, clash with the forces of the Latin emperor Baldwin I of Flanders in the region of Thrace (August or September 1204), Boniface paid fealty to the Latin emperor and received from him Thessalonika as a fief. He also had the Latin emperor ratify the privileges that had been granted to the city by the Byzantine emperors before 1204.
In 1205, Boniface received fealty from different feudal lords of central Greece, Thessaly and Euboea. His bonds with the Latin Empire of Constantinople were strengthened after the marriage of his daughter Agnes with Henry of Flanders (1206-1216), successor of Baldwin I, on 4 February 1207. The vassalage that bound Boniface with the Latin emperor was validated by Henry in the summer of 1207.
After the death of Boniface (September 1207) his minor son Demetrius ascended the throne of Thessalonika (1207-1225). Soon a rebellious movement broke out to the detriment of Demetrius and Henry of Flanders, the Latin emperor of Constantinople. The movement, which was organized by the regent of the kingdom Umberto II de Biandrate and the constable Amadée de Pofoy, aimed at the disengagement of the kingdom from the dominance of the Latin emperor and at the ascent to the throne of Thessalonika of the other son of Boniface, William IV, marquis of Montferrat. A treaty that was signed in May 1209 gave an end to the movement, which, however, achieved its goal in April 1217, when the Latin emperor Peter Courtenay (1216-1217) was convinced by the Lombard claimants to acknowledge to William IV all the rights and obligations of the Latin king of Thessalonika, thus making him the actual ruler.
The kingdom was temporarily threatened, in the beginning of its existence, by the Bulgarian tsar Ioannitzis, who in the summer of 1205 moved towards the region of Adrianople to the west embarking upon predatory and destructive invasions against many Thracian cities (winter-spring of 1205/1206). In the autumn of 1207, Ioannitzis reached the walls of Thessalonika with his troops and started besieging the city. The siege by the Bulgarians had an inglorious end, when Ioannitzis got killed (October 1207). In the summer of 1210, the despot of Epirus Michael I (1204-1215) Komnenos Doukas, having secured the alliance with the Bulgarians, attacked the kingdom capturing many Latins. Thanks to the military intervention of the Latin emperor, Thessalonika was relieved from the menace of Epirus, but many regions in Thessaly, such as Larissa, Pharsala, Velestino and Halmyros surrendered within the two following years.
Until the beginning of 1222, the troops of Epirus had taken the forts of Platamonas and of Serres and were approaching Thessalonika menacingly. In order to ward off the danger of his kingdom falling to the despot of Epirus Theodore I Komnenos Doukas (1215-1230), Demetrius went to Italy in 1222 and asked for help personally. In the summer of the same year, the former regent of Thessalonika and afterwards bailo of the kingdom Umberto II de Biandrate headed a military corps in order to provide military assistance to Thessalonika. In May of the following year a military operation was organized in the West for the defence of Thessalonika. Under the leadership of William IV, marquis of Montferrat, the troops were marshalled in March 1224 in South Italy. However, the military forces of the crusaders arrived with a great delay in Thessaly (1225), whereas the Latin emperor Robert Courtenay (1221-1228), entirely absorbed in the battle against John III Vatatzes (1222-1254), the emperor of Nicaea, did not succeed in sending reinforcements on time for the defence of the kingdom. Finally, in December 1224, Thessalonika surrendered to the despot of Epirus.