Adrianople was an important city in the Byzantine world for all of its history. Thrace was an important province, obviously Constantinople was the most important city in the area, but Adrianople was strategically crucial for maintaining control outside the walls of Constantinople. There are several prominent battles known as The Battle Of Adrianople in history, and this is because is it in such a strategic location.
Adrianople was a city located on the Hebros river, and it was situated along the road system as a key city. The foundation of Constantinople by Emperor Constantine the Great only increased its strategic value. Adrianople became kind of like the main shield of Constantinople, other than of course the mighty Theodosian walls themselves.
THE FIRST BATTLES OF ADRIANOPLE:
The most famous battle of Adrianople is no doubt the fourth century clash between the army of Emperor Valens and the Goths. But there were others as well. The first major battle of Adrianople was actually the victory of Constantine over Licinius in 324AD, which was a key event in Constantine ending the tetrarchy.
In 378, the Goths destroyed the Roman army and killed Valens, the Emperor, one of the worst defeats in Roman history. The Goths had been pushed into imperial territory by the Huns, and served as Roman foederati troops. However, Roman mismanagement and mistreatment of the Goths, combined with their military prowess, brought them into armed confrontation with the Roman army.
According to the Oxford Dictionary of Byzantium: “harsh treatment by Roman officials led the Goths to rebel, and some common people from Adrianople joined them. In 377, (the Eastern Roman Emperor) Valens left Antioch for Constantinople and sought assistance from Gratian, the emperor in the west. Valens led his troops to Adrianople, while Gratian’s army was marching from Gallia to Sirmium.
However, Valens miscalculated. After being briefed with inaccurate reports that the Goths only had 10,000 men, Valens sent his army to attack before Gratian could bring the Western Roman soldiers to the battlefield. The Gothic leader Frittigern offered a peace treaty to Valens, but the Emperor refused to accept. Two thirds of the Roman army was annihilated, a far higher casualty rate than an average defeat. Valens died as well, and Roman Emperors perishing in battle was fairly rare.
Ironically despite the Goths winning the “battle of Adrianople” they actually could not take the city of Adrianople despite attempting to do so. This shows just how stupid Valens was, he could have waited for Gratian and crushed the Goths. Frittigern had to leave Adrianople behind and went raiding across the Balkans even reaching the outskirts of Constantinople. Adrianople was a defendable city, and this is why it would remain important.
ADRIANOPLE AFTER LATE ANTIQUITY:
During some centuries the city is not mentioned frequently by the sources, despite clearly still being fairly important. Byzantine primary sources tend to be more centered on Constantinople than provincial cities. According to the Oxford history of Byzantium “As a bishopric it is known from the end of the fourth century, but its place in the ecclesiastical hierarchy declined from 27th to 40th in the 10th century, despite its growing number of suffregans (bishops). So it does not actually seem like it lost ecclesiastical importance, it grew, just that other places outpaced it.
Adrianople is mentioned in the Taktikon of Escurial, which listed a doux(duke) of Adrianople as second to the doux of Thessaloniki. Essentially it was the third most important city in the Byzantine Balkans. In the 11th and 12th century, there were three usurpers who tried to use the proximity of Adrianople to Constantinople to make a bid to become Emperor. They were Leo Tornikios, Nikephoros Bryennios, and Alexios Branas. However, often Adrianople remained loyal to Constantinople when eastern generals rebelled against the sitting Emperor.
OTHER BATTLES OF ADRIANOPLE:
Due to its important location for the defense of the medieval Roman Empire’s European territories, Adrianople was the location of many battles and was frequently attacked. But it did not fall easily to the enemies of the Romans. The Goths failed to take the city in 378 as I wrote earlier in the article, despite having just eliminated a very large Roman army and killing the Emperor.
In 586, the Avars besieged Adrianople, but they too failed to break through the defenses and could not take the city.
THE THIRD CRUSADE:
The Third Crusade would bring westerners to Adrianople, and it was almost used as a base to destroy the Roman Empire. Isaac II Angelos had been fearful that his enemies in Constantinople would seek to use this foreign army against him. The German Emperor Frederick Barbarossa of the Holy “Roman” Empire went to Philippopolis, and sent word to Isaac to secure his passage across the water from Europe to Asia. However, the German envoys were arrested by Isaac, intended for use as hostages to guarantee the good behavior of Frederick.
However, Frederick did not respond the way Isaac would have hoped. Frederick immediately set out against him, first sending his son also named Frederick to take the town of Didymoteichon. He even wrote back home to his other son Henry back in Germany to send a fleet to Constantinople in order to besiege the Roman capital. He also sent word to ask the Pope to allow for a Crusade to be launched against the Romans. He meant business.
Frederick decided to take Adrianople temporarily, using it as a base. His swift actions forced the Emperor Isaac Angelos to offer a treaty which guaranteed the peace. He returned the German ambassadors, then he gave his own hostages and guaranteed that be would be cooperative. Isaac did secure a provision that the Germans would cross at the Dardanelles instead of the Bosphorus, keeping them away from Constantinople. The mistrust was clear, only 14 years later a Western army would viciously destroy Constantinople. But for now, it was avoided.
FALL OF ADRIANOPLE AND A NEW OTTOMAN CAPITAL:
The Ottomans conquered Adrianople in 1369 after the Ottomans had set about taking what was left of Thrace from the Romans. The Ottomans made it their capital, moving it from Prousa / Bursa in Anatolia. This was the ideal capital city to put one eye on conquering Constantinople. For the same reasons the city was well-situated to protect Constantinople, it was located perfectly to threaten the city. The Sultans army could always march to Constantinople easily. The fall of Adrianople doomed Constantinople.
SOURCES:
The Oxford Dictionary of Byzantium by Alexander Kazhdan
A History of the Crusades, Volume III: The Kingdom of Acre and the Later Crusades by Steven Runciman.
https://www.gordondoherty.co.uk/writeblog/theancientcityofadrianople (for photos)
shadowsofconstantinople.com 
