The Avars had come from the Pannonian Plain in central Europe, a confederation of horse-mounted warriors who built an empire on the back of plundering raids and tribute extracted from neighboring peoples. By the late 6th century they dominated the Slavic tribes north of the Danube, who came to fight either as their auxiliaries or independently as the Avar grip allowed. Byzantine attention was elsewhere. The empire's serious wars were fought against the Sasanian Persians on the eastern frontier, and emperors had little to spare for the Balkans. When the Danube fortresses of Sirmium and Singidunum fell in 582 and 583, the entire region lay open. Slavic tribes began moving south, raiding deeper each year and eventually settling in pockets across what had been Roman territory. Refugees streamed toward whatever cities still held. By the 610s, after Phocas usurped the throne and a catastrophic new war with Persia drained imperial resources, the Danube defense had completely collapsed.

This was not Thessalonica's first attack. An Avar-Slavic siege around 586 had lasted seven days. In 604 a Slavic force of five thousand had attacked the city at night and failed to scale the walls. In 615 a coalition of Slavic tribes under a chief named Chatzon had tried again. Each time the city held, and each time the credit went to Saint Demetrius. The collection of stories called the Miracles of Saint Demetrius became one of the most important sources for early medieval Balkan history precisely because Thessalonians kept writing them down. The first book was compiled around 610, the second around 680, both following the pattern of attributing every successful defense to the saint's intervention. Modern historians read past the miracles to find what actually happened during these sieges. The Thessalonians at the time believed the miracles were the actual happening.

After Chatzon's failure in 615, the Slavs sent emissaries to the Avar khagan. They told him about Thessalonica's wealth. They told him that the city had become the ultimate refuge for everyone fleeing the Avar-Slav advance, with people streaming in from Pannonia, Dacia, Dardania, and the other lost provinces. The khagan was persuaded. He brought an army with technical sophistication far exceeding any of the previous attempts: siege towers, battering rams, the full apparatus of imperial-grade siegecraft. The trouble was that his men did not really know how to use it. The siege tower collapsed and killed its crew. The rams accomplished nothing. The siege dragged on for thirty-three days, far longer than any of the earlier attempts, but the walls held. In the end the khagan agreed to leave in exchange for gold. Before withdrawing, his men burned the churches in the surrounding countryside. The Slavs sold their captured prisoners back to the Thessalonians. Then they all went home.

It is worth pausing on what the Slav peace settlement actually involved. People had been captured during the raids in the countryside around the city. These were farmers, monks, merchants caught traveling, families who had not made it inside the walls in time. The Slavs treated them as commodities. The Thessalonians paid to get their neighbors back. We do not know how many. The Miracles tell us only that the captives were sold to the city, which suggests substantial numbers were exchanged. We also do not know what happened to the rural population the Slavs did not capture, the people who saw their fields burned and churches looted and had nowhere to flee to. They are not in the chronicle. They left no record of their own. The peace that the Thessalonians celebrated as a miracle of Saint Demetrius was for these people, if they were lucky, a return to a destroyed world. For a generation after 617, until the next great siege in 676 to 678, the city would remain at peace with its Slavic neighbors. Some of those neighbors had been captives bought back from slavery. Some of those neighbors had been the captors.