Bands of Celts begin to penetrate southward into the Balkans at the beginning of the third century BCE.

Their superiority rests in part upon their mastery of iron technology, which they use to make both swords and plowshares.

The extent of Celtic settlement is indicated by coins, silverwork, and burial mounds.

Singidunum (now Belgrade), the name of the settlement referred to by the Romans, is partly of Celtic origin.

C. Scribonius Curio, proconsul of Macedonia, had in 75 BCE taken an army as far as the Danube and gained a victory over the inhabitants, who were finally subdued by M. Licinius Crassus, grandson of the triumvir and later also proconsul of Macedonia during the reign of Augustus c. 29 BCE.

The region, however, is not organized as a province until the last years of Augustus' reign; in 6 CE, mention is made of its governor, Caecina Severus (Cassius Dio lv. 29).

As a province, Moesia is under an imperial consular legate (who probably also had control of Achaea and Macedonia).

Bounded on the north by the lower Danube River, on the west by the Drinus (now Drina) River, by the Haemus (Balkan) Mountains on the south, and by the Black Sea on the east, Moesia includes territories of modern-day Southern Serbia (Moesia Superior), Northern Republic of Macedonia, Northern Bulgaria, Romanian Dobrudja, Southern Moldova, and Budjak (Lower Moesia).

Diocletian and later Galerius, fighting along the Danube, have conquered the Iazyges and the Carpi, deporting them in large numbers to the provinces.

The Quadi revolt in outrage and are joined by the Sarmatians, who cross the Danube.

The allied barbarian tribes devastate Moesia and …

The activities of Attila from 435 to 439 are unknown, but he seems to have been engaged in subduing barbarian peoples to the north or east of his dominions.

During six years of peace between the Huns and the Eastern Romans, the latter do not appear to have paid the sums stipulated in the treaty of Margus, so when their forces are occupied in 441 in the west and on the eastern frontier, Attila launches a heavy assault on the Danubian frontier of the Eastern Empire, capturing and razing a number of important cities, including Singidunum (Belgrade).

They have mastered siege technology and are able to capture fortified cities.

The three years following the Hun invasion have been filled with complicated negotiations between Attila and the diplomats of Emperor Theodosius II.

Much information about these diplomatic encounters has been preserved in the fragments of the History of Priscus of Panium.

Roman general (magister militum) Anatolius, responsible for the security of the Eastern frontier, achieves a peace treaty with the Huns in exchange for an annual tribute of twenty-one hundred pounds of gold per year.

Attila demands in the treaty the evacuation of the territory running from Singidunum (Belgrade, in Serbia) three hundred miles east along the Danube to …

Justinian I has sent his general Narses to the rulers of the Heruli, to recruit troops for the campaigns in Italy and Syria.

According to Procopius, the Heruli are a polytheistic society known to practice human sacrifice.

He claims that the Heruli also practice a form of senicide, having a non-relative kill the sick and elderly and burning the remains on a wood pyre.

Following the death of their husband, Heruli women are expected to commit suicide by hanging.

With the ascent of Justinian, Procopius says that the Heruli within the empire converted to Christianity and "adopted a gentler manner of life."

Heruli carry no protective armor in battle save a shield and thick jacket.

Heruli slaves are known to have accompanied them into combat.

Slaves are forbidden from donning a shield until having proven themselves brave on the battlefield.

The Gepids' defeat causes a geopolitical shift in the Pannonian Basin, as it ends the danger represented by the Gepids to the Empire.

The Gepids' utter defeat could have meant the end of their kingdom and its conquest by the Lombards, but Justinian, wanting to maintain an equilibrium in the region, imposes an "eternal peace" that saves the Gepids; it will be observed for ten years, surviving both Thurisind and Audoin.

It may be on this occasion, and not before the war, that Lombards and Gepids sent troops to Narses as part of the peace treaty imposed by Constantinople.

In this interpretation, the small number of Gepid warriors sent could be explained with the heavy losses taken in the war and the resentment felt towards Justinian.

The Emperor also imposes some territorial concessions on Thurisind, obligating him to return Dacia Ripensis and the territory of Singidunum.

To reach a complete peace Thurisind has first to deal with Ildigis, who had found hospitality at Thurisind's court.

Audoin demands yet again to have him turned in, and Justinian joins in the request.

Thurisind, despite his reluctance to resume the war with both Audoin and Justinian, does not want to openly breach the rules of hospitality and thus tries to evade the request by demanding in his turn to have Ostrogotha given to him; in the end, to avoid both openly giving in and at the same time renewing the war, both kings murder their respective guests but keep secret their involvement in the act.

Emperor Justinian I had rebuilt Singidunum in 535, restoring the fortress and city to its former military importance.

The city has seen a half-century of peace, which ends with the arrival of the Avars, who sack the city.

Emperor Maurice decides to end the annual tribute to the Avars, who respond by capturing the cities Singidunum (modern Belgrade) and …