City Reinforced? - We know at the beginning of the siege that the Emperor ordered officers from Illyria and Dalmatia to the city to help organize the defenses. I suspect the officers did not travel alone so they may have brought a small party of soldiers with them. It is very possible that over the years that the garrison received periodic small amounts of supplies and reinforcements from neighboring Roman towns.
The Avars may have controlled the countryside with their cavalry, but they faced the task of keeping thousands of their troops supplied with food for years.
What Fighting Took Place? - Unfortunately there are no records of battles with the Avars, but I am almost 100% sure that the opposing forces did not sit and stare at each other doing nothing for nearly three years.
The Avars were equestrian warrior nomads who appeared to have no ability or interest in constructing siege weapons to bring down the city walls. A siege causing starvation is the best they could manage.
The Avar Khagan would have faced rebellion in his ranks from restless soldiers with nothing to do for months on end. The Khagan initially would have tried attacks on the city testing the defenses. As the Romans held firm the Khagan would probably have planned a regular series of smaller day and night probes of the city walls hoping to catch the Romans off guard.
Being short on troops the Roman garrison may not have responded to the Avar attacks beyond cheering from the walls at the defeat of the enemy. If neighboring Roman garrisons had tried to help resupply Sirmium by river or land at night there could have been a number of small actions against the Avars.
|The Nearby Roman Fortress of Singidunum.|
What We Do Know - Historian Edward Gibbon says the Avar Khagan Bayan I marched with his men to the Roman Fortress of Singidunum which controlled the conflux of the Danube and Sava Rivers. An Avar fleet of large boats had been constructed to transport bridge making materials up the rivers.
The Roman commander of this strong garrison challenged the Khagan as to his intentions saying his movements were a violation of the peace treaty. The Khagan swore oaths that he was not hostile to the Empire saying, "If I violate my oath may I myself, and the last of my nation, perish by the sword." The Khagan claimed he was simply moving to attack Slav invaders who had refused to pay the Avars their annual tribute.
The Bishop of Singidunum held services with the rather disrespectful Avars who gave even more oaths of friendship.
The Avar Khagan marched with his men from Singidunum following the right bank of the Sava River to Sirmium.
Once the city was besieged Bayan said, "Inform the Emperor that Sirmium is invested on every side. Advise his prudence to withdraw the citizens and their effects, and to resign a city which it is now impossible to relieve or defend."
|Byzantine Infantry 6th C. AD|
The city at the time lacked a large enough garrison and unprepared to withstand a siege, as most of the Byzantine forces were engaged in the east against Sassanid Persia. The Byzantine Emperor Tiberius II tried to forestall the Avar attack by diplomatic means, but when the Khagan′s ambassador demanded the surrender of the city Tiberius said in reply, "I would sooner give your master one of my two daughters to wife than I would of my own free will surrender Sirmium."
An interesting side note. The Avar ambassador was returning from Constantinople with his Roman escort when the entire party was killed by Slav pillagers. This casually mentioned event in Byzantine accounts tells us a great deal about the lawless conditions in the open countryside of the Roman Balkan provinces.
Tiberius's armies were committed in Persia and Italy, but managed to send in a few officers from Dalmatia and Illyria to oversee the city's defenses. The Roman commander Theognis met with the Khagan on the islands of Casia and Carbonaria, but the negotiations proved to be fruitless.
Despite the relative weakness of the garrison, the city resisted for almost three years. With no end in sight for the Persian War 