After the fall of Constantinople to the crusaders in 1204, Boniface of Montferrat, the leader of the crusade, was expected by both the Crusaders and the defeated Byzantines to become the new emperor. However, the Venetians felt that Boniface was too closely tied to the Byzantine Empire, as his brother Conrad had married into the Byzantine imperial family. The Venetians wanted an emperor whom they could control more easily, and with their influence, Baldwin of Flanders was elected as emperor of the new Latin Empire.

Boniface reluctantly accepted this, and set out to conquer Thessalonica, the second-largest Byzantine city after Constantinople. At first he had to compete with Emperor Baldwin, who also wanted the city. He then went on to capture the city later in 1204 and set up a kingdom there, subordinate to Baldwin, although the title of "king" was never officially used.

In 1204–05, Boniface was able to extend his rule south into Greece, advancing through Thessaly, Boeotia, Euboea, and Attica Boniface's rule lasted less than two years before he was ambushed by Tsar Kaloyan of Bulgaria and killed on September 4, 1207. The kingdom passed to Boniface's son Demetrius, who was still a baby, so actual power was held by various minor nobles of Lombard origin.