Siege of Thessalonica ( 617 ) The Siege of Thessalonica in 617 or 618 was an unsuccessful siege of the city of Thessalonica , the major Byzantine stronghold in the region , by the Avars and the Slavic tribes ( " Sclaveni " ) who had settled in the city ' s vicinity . The attack was the last and best - organized attempt by the Avars to take the city . It lasted for 33 days and involved the use of siege engines , but in the end failed . The main source for these events are the " Miracles of Saint Demetrius " , named after Thessalonica ' s patron saint , Saint Demetrius . Background . In the last third of the 6th century , the Byzantine Balkans were threatened by large - scale raids of the Avars , based in the Pannonian Plain , and their Slavic allies , based north of the Danube , which marked the border of the Byzantine Empire . The Byzantines , focusing on their eastern border , where they faced the Sassanid Persians in a protracted war , were unable to maintain an effective defence of the region : following the fall of Sirmium in 582 and of Singidunum in the year after , the Balkans lay open to Avar raiding . Along with the Avars , the breach in the Danube " limes " allowed the Slavic tribes to raid further and further south into Greece , and to begin a gradual process of settlement in these areas , the extent , chronology and other details of which are much debated . During these raids , probably in 586 ( although 597 is a possible alternative date ) , Thessalonica , the most important city in the Balkans except for the imperial capital , Constantinople , was besieged by the Avars and their Slavic auxiliaries for seven days , as described in the " Miracles of Saint Demetrius " , a collection of miracles attributed to the city ' s patron saint in two books , one written ca . 610 and the other around 680 . After peace had been won with the Persians in the East in 591 , Byzantine emperor Maurice and his generals were able to drive back the Slavs and the Avars in a series of campaigns . However , Maurice ' s victories ultimately failed to restore the stability of the Danube " limes " due to the rebellion of the Danube army in 602 , which led to Maurice ' s deposition and murder , and the accession of the usurper Phocas . The renewal of war with Persia meant the rapid and complete collapse of the Danube frontier in the first decades of the 7th century , as imperial forces were withdrawn to the East . Phocas and his successor Heraclius bought off peace with the Avars through annual tributes , but the Slavs once again had a free hand in raiding the Balkans , and in 604 , a force of 5 , 000 men suddenly attacked Thessalonica at night , but failed to scale the city walls , the reason for this failure was that the city was fortified . Siege and aftermath . It was not until the 610s , however , that the Avars and the Slavs renewed their offensive ; a number of cities seems to have fallen and / or abandoned at this time , with many refugees streaming south . In 615 , a coalition of Slavic tribes under a certain chief Chatzon , apparently independent of the Avars , attacked Thessalonica , but failed . Following this failure , the Slavs turned to the Avars , and sent emissaries to the " khagan " , enticing him with promises of great riches to be found in the city , and the fact that Thessalonica was the ultimate refuge for the Byzantines " from the Danubian regions , Pannonia , Dacia , Dardania and the other provinces and cities " fleeing the Avar and Slavic raids . The Avar attack materialized in 617 ( or possibly 618 ) , as they needed time to mobilize their various subject tribes . According to the narrative of the " Miracles of Saint Demetrius " , the attack was unexpected : the Avars first sent scouts who captured anyone they caught outside the city walls . The " khagan " with the bulk of his forces , including heavy siege engines , catapults , battering ram s and siege tower s , arrived a few days later . Emperor Heraclius , surprised by the Avar attack and heavily committed against the Persians , was unable to send any help ; except for a few supply ships that arrived on time , the city was forced to rely on its own forces . Although the technical sophistication of the besiegers was unprecedented , they were yet apparently unable to make full use of it due to inexperience : a siege tower collapsed and killed its crew , while the battering rams proved ineffective against the city walls . The siege was far better organized than the previous attempts , however , and dragged on for 33 days . In the end , the " khagan " reached a negotiated settlement with the Thessalonians : he departed in exchange for gold , but not before burning the churches of the surrounding countryside . The Slavs , on the other hand , sold their captives to the Thessalonians . For a generation , until the great Slavic siege of ca . 676 – 678 , Thessalonica would remain in peace with its Slavic neighbours .