Early Slavs began mass migrating to Southeastern Europe between the first half of the 6th and 7th century in the Early Middle Ages. The rapid demographic spread of the Slavs was followed by a population exchange, mixing and language shift to and from Slavic.
The settlement was facilitated by the substantial decrease of the Southeastern European population during the Plague of Justinian. Another reason was the Late Antique Little Ice Age from 536 to around 660 CE and the series of wars between the Sasanian Empire and the steppe nomads against the Eastern Roman Empire. After the arrival of the Pannonian Avars in the mid-6th century, they continued to conduct incursions into Roman territory, often independently of Avar's influence. After the failed siege of Constantinople in the summer of 626, and successful revolt against the Avars, they remained in the wider Southeast Europe area after they had settled the Byzantine provinces south of the Sava and Danube rivers, from the Adriatic Sea to the Aegean and Black Sea.
Exhausted by several factors and reduced to the coastal parts of the Balkans, Byzantium was not able to wage war on two fronts and regain its lost territories, so it reconciled with the establishment of Sklavinias and created an alliance with them against the Avar and Bulgar Khaganates.
Background
Before the great migration period, the population of the Southeast Europe was composed of Ancient Greeks, Illyrians and Thracians who had been Romanized and Hellenized, as well as of Roman Imperial subjects. Communities of Bastarnae, Sciri, Heruli, Gepids and Lombards were also present by the mid-6th century. After the destructive campaigns of Attila the Hun and the Goths, who were previously foederati, which resulted in the fall of the Western Roman Empire (c. 476 CE), Eastern Roman Emperor Justinian I began the reconstruction of fortresses, cities, and churches. However, the Plague of Justinian (from 541–549 until the mid-8th century) decimated the native population, resulting in the weakening of the Pannonian and Danubian Limes. Various factors, including the Late Antique Little Ice Age and population pressure, pushed the migration of the Early Slavs, some of whom since the mid-6th century were also led by the Pannonian Avars (while others led the conquest against the Avars).Wright, David Curtis (1997). "The Hsiung-Nu-Hun Equation Revisited". Eurasian Studies Yearbook. 69: 77–112. The adoption of other medieval European cultural customs, especially related to agriculture, also caused population growth among the Slavs.
Early Slavs could have been sporadically present in the Carpathian Basin during the time of Sarmatian Iazyges (and related to Limigantes). They possibly participated in the campaigns of the Huns and of various Germanic tribes from the end of the 5th century, although there's no record in historical sources, the evidence being recorded words "medos, kamos, strava" in a Hunnic camp.
History
The Slavs who settled in Southeast Europe comprised two groups: the Antae and the Sclaveni. The first certain Slavic raids date to the early 6th century during the time of the Eastern Roman Emperor Justin I (), coinciding with the end of the Vitalian revolt of 511–518. Procopius recorded that in 518 a large army of the Antae, "who dwell close to the Sclaveni", crossed the Danube River into Roman territory, but suffered a bad defeat by magister militum per Thraciam Germanus. In 536, some 1,600 horsemen of Slavs, Huns and Antae arrived in Italy as Byzantine reinforcement to rescue Belisarius, serving also as horsearchers. Raids continued with ever-faster and stronger incursions during the time of Justinian I (), with Procopius recording that the whole of Illyricum and Thrace (killing its magister militum Chilbudius) was pillaged almost every year by Huns, Sclaveni, and Antae, who with each incursion did enormous damage to the native Roman population, making the region a "Scythian desert". Jordanes considered the devastation wrought by Slavs to be a divine punishment for the Roman sins. They also possibly participated in the devastating 539/540 winter raid of the Huns which covered lands from the Adriatic coast to the Constantinople. Around the 540s the Sclaveni accepted the exiled Lombard prince Ildigis/Hildigis son of Risiulfus who was escaping from Audoin-Thurisind's conflict and helped him to organise his own army of 6,000 Gepids and Sclaveni which temporarily supported Thurisind's case, and Totila of Goths in Italy, but the exact location of the Slavs in question remains a matter of scholarly debate (but most probably in the Carpathian Basin near the Danubian frontier).
For a great throng of the barbarians, Sclaveni, had, as it happened, recently crossed the river Ister, plundered the adjoining country and enslaved a very great number of Romans ... Huns and Antae and Sclaveni had already made the crossing many times and done irreparable harm to the Romans ... At about this time an army of Sclaveni crossed the river Ister [Danube] and spread desolation throughout the whole of Illyricum as far as Epidamnus, killing or enslaving all who came in their way, young and old alike and plundering their property. And they had already succeeded in capturing numerous strongholds of that region, which were then quite undefended, but which previously had been reputed to be strong places, and they continued to roam about searching out everything at their own pleasure. And the commanders of the Illyrians kept following them with an army of 15,000 men, without, however, having the courage to get close to the enemy.
At about this time an army of Sclaveni amounting to not more than 3,000 crossed the Ister River without encountering any opposition, advanced immediately to the Hebrus River, which they crossed with no difficulty, and then split into two parts. Now the one section of them contained 1,800 men, while the other comprised the remainder. And although the two sections were thus separated from each other, the commanders of the Roman army, upon engaging with them, both in Illyricum and in Thrace, were defeated unexpectedly, and some of them were killed on the field of battle, while others saved themselves by a disorderly flight.
Now after all the generals had fared thus at the hands of the two barbarian armies, though they were far inferior to the Roman forces in number, one section of the enemy engaged with Asbadus. This man was a guard of the Emperor Justinian, since he served among the candidati as they are called, and he was also commander of the cavalry cohorts which from ancient times have been stationed at Tzurullum, the fortress in Thrace, a numerous body of the best troops. These too the Sclaveni routed with no trouble, and they slew the most of them in a most disgraceful flight ... Having accomplished these things, they turned to plunder all the towns, both of Thrace and of Illyricum, in comparative security; and both armies captured many fortresses by siege ... Then those who had defeated Asbadus plundered everything in order as far as the sea and captured by storm a city on the coast named Topirus ... Then they slew all the men immediately, to the number of 15,000, took all the valuables as plunder, and reduced the children and women to slavery. Before this, however, they had spared no age, but both these aud the other group, since the time when they fell upon the land of the Romans, had been killing all who fell in their way, young and old alike, so that the whole land inhabited by the Illyrians and Thracians came to be everywhere filled with unburied corpses ... Thus did the Sclaveni consistently destroy those who fell in their way. But from this time onward both these and those of the other group, being as it were drunk with the great quantity of blood they had shed, saw fit to make prisoners of some who fell into their hands, and consequently they were taking with them countless thousands of prisoners when they all departed on the homeward way.
At one point in the late 530s or early 540s, the Sclaveni and Antae "became hostile to one another and engaged in a battle, in which it so fell out that the Antae were defeated by their opponents". In 545 Justinian I managed to make an alliance with the Antae to stop Hunnic intrusions into the Lower Danube area, but this caused more Sclaveni intrusions from the Middle Danube region. According to Procopius, around the summer of 550 a massive number of Sclaveni "such as never before was known arrived on Roman soil, having crossed the Ister River and come to the vicinity of Naissus", planning to capture by siege Thessalonica and near cities. Hearing that Germanus was organizing large forces in Serdica southeast of Naissus (initially with plans for a campaign against Totila's Goths in Italy, but now devoted to repel Sclaveni's invasion; meanwhile Narses had to deal with Kutrigurs around Philippopolis), the Sclaveni abandoned their initial plans and crossed the Dinaric Alps mountain ranges of Illyricum and came into the province of Dalmatia. Germanus soon died, and the army now led by his son-in-law John (nephew of Vitalian) and son Justinian was ordered to start marching to Dalmatia, pass the winter in Salona and then move from there to Ravenna. By the time they entered the province of Dalmatia, the first group of Sclaveni was joined by a second group who also crossed the Danube, and "overrun the Roman domain with complete freedom ... divide themselves into three groups and wrought irreparable damage ... not merely plundering that country by sudden raids, but actually spending the winter as if in their own land and having no fear of the enemy", disabling Roman armies movement to Ravenna but not engaging in battle. Procopius reported that some thought that the Sclaveni were invited with large sums of money by Totila to divert emperor's forces from attacking the Goths in Italy by land.
Eventually, emperor Justinian decided to assemble a large army led by the commanders John, Narses, Constantianus, Aratius, Justin (eldest son of Germanus), and supreme commander Scholasticus (imperial eunuch). They clashed in spring 551 with one of the three groups of Slavs near Adrianopolis in Thrace, where "Romans were decisively vanquished. In that battle many of the best soldiers perished, and the generals came within a little of falling into the hands of the enemy, succeeding only with difficulty in making their escape with the remnant of the army and thus saving themselves, each as best he could", the Sclaveni captured the standard of Constantianus, went on to plunder Astica and reached the Anastasian Wall, where Roman forces managed a victory over a part of the barbarian forces, retrieving the standard and rescued many Roman captives, but nevertheless the Sclaveni departed with large looting. In the same year or early 552, another throng of Sclaveni and 12,000 Kutrigurs attacked Illyricum "and inflicted sufferings there not easily described", helped twice by the Gepids to cross the Danube (as Gepids controlled the river passages of the Middle Danube, Sava and Drava, including territory around Sirmium, and in doing so enabled a route avoiding defensive forts on the limes and enter Upper Moesia, Illyricum, Thrace up to Constantinople):