Kaloyan of Bulgaria destroyed the field army of the Latin Empire outside Adrianople on 14 April 1205 and captured the emperor Baldwin I, who died in Bulgarian captivity; the empire founded a year earlier never replaced the knights it lost.
The Battle of Adrianople of 14 April 1205 was fought between the Latin Empire founded by the Fourth Crusade and the Bulgarian tsar Kaloyan. It is not the Battle of Adrianople of 378, in which the Goths destroyed the emperor Valens on the same Thracian plain; the two are separated by more than eight centuries and are constantly confused because the city kept its name. This one killed no emperor outright. It did something the Latins found harder to survive: it took their army and their ruler in an afternoon.
Baldwin I had been crowned in Constantinople less than a year before. When the Greek population of Thrace rebelled and called in Kaloyan, Baldwin marched on Adrianople with whatever knights were not garrisoning the new conquests. Kaloyan used his Cuman allies exactly as steppe cavalry had always been used against western knights: a shower of arrows, a feigned flight, and prepared ground at the end of the pursuit.
The crusaders who stormed Constantinople in April 1204 partitioned an empire they had not conquered. Baldwin of Flanders took the imperial title and a strip of Thrace, Boniface of Montferrat took Thessalonica, the Venetians took the ports, and the Greek aristocracy of the provinces was expected to accept new lords who held their lands by conquest and would not confirm their privileges. In the spring of 1205 the towns of Thrace rose, beginning with Adrianople, and drove out or killed the Latin garrisons.
Kaloyan had already offered the Latins an alliance and been refused in terms that made an enemy of him: his ambassadors were told he was a subject rebelling against his rightful lords. He had spent a decade making Bulgaria a power again, had taken a crown from a papal legate at Tarnovo in November 1204, and he had the Cuman confederation of the Danube steppe on retainer. When the Thracian Greeks asked for help, he had both the motive and the cavalry.
Baldwin invested Adrianople in late March with a force too small for the work: siege engines against a walled city, a mine under one tower, and not enough men to close the circuit. Kaloyan reached the area on about 10 April and camped some twenty-five kilometres off. On 13 April a Cuman raiding party rode up to the Latin camp, shot into it, and withdrew when the knights came out; the pursuit went several kilometres and achieved nothing.
That was the rehearsal. Baldwin called a council, and it agreed the obvious discipline: no one was to break ranks, whatever provocation the horse-archers offered. Villehardouin, who was there as marshal, records the order and records that it was given twice.
On the morning of 14 April, Easter Thursday by the Latin reckoning, the Cumans came again and shot into the camp while the army was at its devotions. Count Louis of Blois, who had been ill and was carried to his horse, treated the interruption as a sacrilege and charged with his own knights. Baldwin followed him rather than leave him unsupported, with perhaps two hundred more. The Cumans fled for about four kilometres, the standard distance of a steppe feint, and the pursuit strung itself out across country the Bulgarians had prepared.
Where the ground broke, the Latins met pits dug and covered to catch horses, and the Cumans turned. Kaloyan's Bulgarian and Vlach infantry came out of the ravines behind them, and the fighting became what heavy cavalry most feared: dismounted knights, in small groups, surrounded on foot. Villehardouin and Niketas Choniates agree on the method used to unhorse them, with ropes and hooked poles. Louis of Blois was killed with Renaud de Montmirail, Stephen du Perche and most of the knights who had ridden out. Baldwin was pulled down and taken alive.
In the camp the marshal and the doge held the remaining troops together. Enrico Dandolo, blind and past ninety, organised a retreat that began after nightfall and did not stop until it reached the sea at Rodosto. He died some weeks later at Constantinople, worn out by it, and was buried in Hagia Sophia.
Baldwin was taken to Tarnovo and imprisoned in the Tsarevets fortress; the tower shown to visitors there is still called Baldwin's Tower, though the structure was rebuilt in 1930. What happened to him afterwards is genuinely unrecoverable. Kaloyan wrote to Pope Innocent III that the emperor had died in captivity, and his brother Henry accepted it and took the crown; nothing in the record fixes the date or the cause.
The chroniclers then went to work on the gap. Niketas Choniates has him tortured and killed; George Akropolites has his skull made into a drinking cup, which is what Byzantine writers had said of Nikephoros I and Khan Krum four centuries earlier and is a literary formula rather than evidence; a later western tale has him executed for making advances to Kaloyan's wife. Henry of Flanders, who had every reason to blacken Kaloyan, reported instead that his brother had been treated with respect. A man who turned up in Flanders in 1225 claiming to be Baldwin was hanged as an impostor, which is its own comment on how unfinished the story felt.
The Latin Empire never rebuilt the force it lost on 14 April. Its strength had always been a few thousand western knights who could not be replaced from the country they ruled, and after Adrianople it fought its wars with hired men, Venetian ships and whatever crusading parties passed through. Henry of Flanders, regent from 1205 and emperor from 1206, proved much the better ruler of the two brothers — he beat the Bulgarians in the field, made terms with the Greek aristocracy his brother had alienated, and held the thing together until his death in 1216 — but he was managing a shortage rather than ending it.
The beneficiary was Nicaea. While the Latins spent their field army in Thrace, Theodore Laskaris was consolidating the strongest of the Greek successor states in Asia, and it was his line's general who walked into Constantinople in 1261. Kaloyan gained less than the victory promised: he took Serres, ravaged Thrace and Macedonia, adopted the title Romanoktonos — Roman-slayer, a deliberate answer to Basil II's Bulgar-slayer — and was dead outside Thessalonica in October 1207, of pleurisy by one account and by a lance from Saint Demetrius in another.