A battle between the Bulgarians under their ruler, Kalojan, and an army of Franks under Baldwin I, the new Latin emperor of Constantinople, who was trying to suppress a revolt by the Greeks in Thrace.
Without waiting for the return of all the Franks then campaigning in Asia Minor, Baldwin led a small army (140 knights and their followers) from Constantinople (mod. Istanbul, Turkey), accompanied by Enrico Dandolo, doge of Venice, and Louis of Blois, duke of Nicaea, and joined up with a small force under Geoffrey of Villehardouin. The Frankish army took up a position outside Adrianople (mod. Edirne, Turkey) on 29 March 1205 but was able to blockade only two gates and was very short of supplies. Kalojan arrived with a large army including 14,000 Cumans to help the besieged (13 April). The fighting on the first day (14 April) was inconclusive, but the crusaders suffered many losses when the Cumans feigned flight to draw their enemy into pursuit. That evening a war council issued strict orders that the army should await the enemy attack without moving. On 15 April the Cumans rode right up to the Franks’ lines, and, despite the agreed battle plan, Louis of Blois led his men in pursuit of them and called on Baldwin to follow. The Franks were picked off individually, and the battle ended in a disastrous defeat, with Louis dead and the emperor captive. The survivors joined the rearguard under Geoffrey of Villehardouin, who led a disciplined retreat to Rodosto.
The news of the battle provoked a mass flight of Latins from Constantinople (7,000 according to Villehardouin) and encouraged further revolts by the Greeks. The Latin Empire was seriously weakened by the loss of manpower and the interregnum, which lasted until news of the death of Emperor Baldwin in captivity reached Constantinople.