Answer: Slavs and Avars
In 550 CE, a group of barbarian invaders from eastern Europe named the Slavs began to raid the Balkan territory of the Byzantine Empire. To turn back these raids, the Byzantines arranged for another barbarian group, the Avars, to attack the Slavs.
These barbarian tribes were often at war with each other over territory and power. The Byzantines hoped to take advantage of this. The Avars lived in the Caucasus region and were related to the Huns. The Byzantines hoped that the Slavs would be too busy fighting the Avars to raid the Balkans. However, this plan backfired horribly.
Avars and Sassanids
Not only was the Byzantine army fighting the Avars mutiny, rebels in Constantinople also rioted and killed the emperor. Shortly afterwards, the new emperor was also killed by a competitor for the throne, Heraclius.
Arabs
After five years of peace, a new enemy threatened the empire. The Arabs managed to defeat Byzantine armies in Syria and Palestine, and after Heraclius died, they took control of Egypt.
Arab armies swept across Persia and Byzantine territories in North Africa and East Asia. They then turned their attention toward capturing Constantinople.
The Arabs attacked the city twice, in 674 CE and in 717 CE, and were defeated both times. The Byzantines were greatly aided by a weapon called Greek Fire. This weapon spewed fire at attacking ships, causing them to burst into flames.
Bulgars
In the early 600s, the Bulgar people united to form a powerful kingdom that stretched along the Danube in the Balkans. This group mixed with the Slavic people.
The Bulgarian Kingdom attacked the Byzantine Empire several times, which caused the Byzantine emperor to lead an army against them in 811 CE. The Bulgars crushed the Byzantine troops, killed the emperor, and two years later, attacked Constantinople.
