In 1204 the Crusaders of the Fourth Crusade conquered Constantinople and founded the Latin Empire on the ruins of Byzantium, crowning Baldwin, Count of Flanders (IX) and Hainault (VI), as the Emperor of Constantinople.
The "Greeks" (in this case meaning the Eastern Orthodox former subjects of Byzantium, as opposed to the Roman Catholic "Latins") sent envoys to Kaloyan, promising to make him emperor, if he would provide them with protection. The Crusaders, together with the Venetians under Doge Enrico Dandolo, challenged Kaloyan, and on March 29, 1205, laid siege to Adrianople, which Kaloyan had placed under his protection. Kaloyan hurried to Adrianople with a large army composed of Bulgars (Bulgarians), Vlachs (Wallachians), and 14,000 Comans (Koumanian mercenaries). On Thursday, April 14, 1205, the Crusaders faced Kaloyan outside of the city of Adrianople in what came to be called the Battle of Odrin. Kaloyan soundly defeated the Crusaders and took Emperor Baldwin prisoner. Baldwin languished in prison in the Bulgarian capital of Turnovo until his death on June 11, 1205, despite repeated threats from the pope of a crusade against Bulgaria.
After the Battle of Odrin, Kaloyan began a merciless war on both the Greeks and Latins. By the summer of 1205, Kaloyan held everything on the western side of the straits of St. George except for Rodosto and Selymbria. Meanwhile Theodore I Lascaris, Emperor of Nicaea, had gained considerable territory to the east of Constantinople, following its capture by the Crusaders.
Kaloyan pursued Boniface, the Marquis of Montferrat and leader of the Fourth Crusade, destroying Seres and Philippopolis. On January 31, 1206, Kaloyan's forces slaughtered 120 French knights near Rusium, and shortly thereafter took Arpos (called Naples or Napoli by the Crusaders) by force, killed and enslaved its inhabitants, and destroyed the city. When the news of the fate of Arpos (Napoli) reached the city of Rodosto, the Venetians, and the Flemish and French soldiers that were guarding the city fled, and the remaining inhabitants surrendered to Kaloyan. He then captured the cities of Panedor, Heraclea (a seaport owned by the Venetians), Daonium, Tzurulum, and Athyra. The Crusaders were left with only Constantinople and the two nearby cities of Bizye and Selymbria.
The Crusaders then had a number of small victories, and began to attack the territories held by Theodore I Lascaris, Emperor of Nicaea. At the beginning of March 1207 Kaloyan and Theodore I Lascaris formed an alliance against the Crusaders. In April of 1207 Kaloyan laid siege to Adrianople, but after a month the Comans left, and Kaloyan ended the siege. In June 1207 Theodore I Lascaris agreed to a two year truce with the Crusaders, who were now under the command of Baldwin's brother Henry. On September 4, 1207, a group of Bulgarians attacked Boniface of Montferrat, killed him and sent his head to Kaloyan. Shortly thereafter Kaloyan was killed in his sleep by Manastre, the leader of the Koumanian mercenaries in his army, during the siege of Solun, Thessalonica.