By about 580, the Avar Khagan Bayan I had established supremacy over most of the Slavic, Bulgar and Germanic tribes living in Pannonia and the Carpathian Basin.
When the Byzantine Empire was unable to pay subsidies or hire Avar mercenaries, the Avars raided their Balkan territories.
According to Menander, Bayan commanded an army of 10,000 Kutrigur Bulgars and sacked Dalmatia in 568, effectively cutting the Byzantine terrestrial link with North Italy and Western Europe.
 By 582, the Avars had captured Sirmium, an important fort in Pannonia.
When the Byzantines refused to increase the stipend amount as requested by Bayan's son and successor Bayan II (from 584), the Avars proceeded to capture Singidunum and Viminacium.
They suffered setbacks, however, during Maurice's Balkan campaigns in the 590s.

By 600 the Avars had established a nomadic empire ruling over a multitude of peoples and stretching from modern Austria in the west to the Pontic–Caspian steppe in the east.
After being defeated at the Battles of Viminacium in their homeland, some Avars defected to the Byzantines in 602 but Emperor Maurice decided not to return home as was customary.
He maintained his army camp beyond the Danube throughout the winter but the hardship caused the army to revolt, giving the Avars a desperately needed respite and they attempted an invasion of northern Italy in 610.
The Byzantine civil war prompted a Persian invasion in the Byzantine–Sasanian War of 602–628 and after 615, the Avars enjoyed a free hand in the undefended Balkans.

While negotiating with Emperor Heraclius beneath the walls of Constantinople in 617, the Avars launched a surprise attack.

While they were unable to capture the city centre they pillaged the suburbs of the city and took 270,000 captives.
Payments in gold and goods to the Avars reached the record sum of 200,000 solidi shortly before 626.
In 626, the Avars cooperated with the Sassanid force in the failed siege of 626.
Following this defeat, the political and military power of the Avars declined.
Byzantine and Frankish sources documented a war between the Avars and their western Slav clients, the Wends.

Each year, the Huns [Avars] came to the Slavs, to spend the winter with them; then they took the wives and daughters of the Slavs and slept with them, and among the other mistreatments [already mentioned] the Slavs were also forced to pay levies to the Huns.
 But the sons of the Huns, who were [then] raised with the wives and daughters of these Wends could not finally endure this oppression anymore and refused obedience to the Huns and began, as already mentioned, a rebellion.
 When now the Wendish army went against the Huns, the [aforementioned] merchant Samo accompanied the same.
And so the Samo’s bravery proved itself in wonderful ways and a huge mass of Huns fell to the sword of the Wends.

— Chronicle of Fredegar, Book IV, Section 48, written circa 642
In the 630s, Samo, the ruler of the first historically known Slavic polity known as Samo's Tribal Union or Samo's realm, increased his authority over lands to the north and west of the Khaganate at the expense of the Avars, ruling until his death in 658.

The Chronicle of Fredegar recorded that during Samo's rebellion in 631AD, 9,000 Bulgars led by Alciocus left Pannonia to modern-day Bavaria where Dagobert I massacred most of them.
The remaining 700 joined the Wends.
At about the time of Samo's realm, the Kubrat of the Dulo clan led a successful uprising to end Avar authority over the Pannonian Plain; he established Old Great Bulgaria, or Patria Onoguria, "the homeland of Onogurs".
The civil war, possibly a succession struggle in Onoguria between the joint Kutrigur and Utigur forces, raged from 631 to 632.
The power of the Avars' Kutrigur forces was shattered and the Avars came under the control of Great Bulgaria.
At the same time, according to Constantine VII's work De Administrando Imperio (10th century), a group of Croats separated from the White Croats who lived in White Croatia and arrived by their own will, or were called by the Byzantine Emperor Heraclius
By about 580, the Avar Khagan Bayan I had established supremacy over most of the Slavic, Bulgar and Germanic tribes living in Pannonia and the Carpathian Basin. When the Byzantine Empire was unable to pay subsidies or hire Avar mercenaries, the Avars raided their Balkan territories. According to Menander, Bayan commanded an army of 10,000 Kutrigur Bulgars and sacked Dalmatia in 568, effectively cutting the Byzantine terrestrial link with North Italy and Western Europe.  By 582, the Avars had captured Sirmium, an important fort in Pannonia. When the Byzantines refused to increase the stipend amount as requested by Bayan's son and successor Bayan II (from 584), the Avars proceeded to capture Singidunum and Viminacium. They suffered setbacks, however, during Maurice's Balkan campaigns in the 590s.
By 600 the Avars had established a nomadic empire ruling over a multitude of peoples and stretching from modern Austria in the west to the Pontic–Caspian steppe in the east. After being defeated at the Battles of Viminacium in their homeland, some Avars defected to the Byzantines in 602 but Emperor Maurice decided not to return home as was customary. He maintained his army camp beyond the Danube throughout the winter but the hardship caused the army to revolt, giving the Avars a desperately needed respite and they attempted an invasion of northern Italy in 610. The Byzantine civil war prompted a Persian invasion in the Byzantine–Sasanian War of 602–628 and after 615, the Avars enjoyed a free hand in the undefended Balkans.
While negotiating with Emperor Heraclius beneath the walls of Constantinople in 617, the Avars launched a surprise attack. 
While they were unable to capture the city centre they pillaged the suburbs of the city and took 270,000 captives. Payments in gold and goods to the Avars reached the record sum of 200,000 solidi shortly before 626. In 626, the Avars cooperated with the Sassanid force in the failed siege of 626. Following this defeat, the political and military power of the Avars declined. Byzantine and Frankish sources documented a war between the Avars and their western Slav clients, the Wends.
Each year, the Huns [Avars] came to the Slavs, to spend the winter with them; then they took the wives and daughters of the Slavs and slept with them, and among the other mistreatments [already mentioned] the Slavs were also forced to pay levies to the Huns.  But the sons of the Huns, who were [then] raised with the wives and daughters of these Wends could not finally endure this oppression anymore and refused obedience to the Huns and began, as already mentioned, a rebellion.  When now the Wendish army went against the Huns, the [aforementioned] merchant Samo accompanied the same. And so the Samo’s bravery proved itself in wonderful ways and a huge mass of Huns fell to the sword of the Wends. 
— Chronicle of Fredegar, Book IV, Section 48, written circa 642
In the 630s, Samo, the ruler of the first historically known Slavic polity known as Samo's Tribal Union or Samo's realm, increased his authority over lands to the north and west of the Khaganate at the expense of the Avars, ruling until his death in 658.
The Chronicle of Fredegar recorded that during Samo's rebellion in 631AD, 9,000 Bulgars led by Alciocus left Pannonia to modern-day Bavaria where Dagobert I massacred most of them. The remaining 700 joined the Wends. At about the time of Samo's realm, the Kubrat of the Dulo clan led a successful uprising to end Avar authority over the Pannonian Plain; he established Old Great Bulgaria, or Patria Onoguria, "the homeland of Onogurs". The civil war, possibly a succession struggle in Onoguria between the joint Kutrigur and Utigur forces, raged from 631 to 632. The power of the Avars' Kutrigur forces was shattered and the Avars came under the control of Great Bulgaria. At the same time, according to Constantine VII's work De Administrando Imperio (10th century), a group of Croats separated from the White Croats who lived in White Croatia and arrived by their own will, or were called by the Byzantine Emperor Heraclius
Pannonian Avars