1. Lombards: The Lombards were a Germanic tribe that migrated from Eastern Europe into the Italian peninsula. They took advantage of the weakened state of the Byzantine Empire to establish their kingdom in northern Italy in the late 6th century. The Lombards posed a constant threat to Byzantine territories in Italy, leading to prolonged conflicts and territorial losses for the empire.
2. Avars: The Avars were a nomadic group of Central Asian origin who moved westward during the 6th century. They established a powerful empire in the region north of the Danube River, which frequently clashed with the Byzantine Empire. The Avars were skilled cavalrymen and often launched devastating raids into Byzantine territory, causing widespread destruction.
3. Persians (Sassanids): The Sassanid Empire was a powerful rival to the Byzantine Empire in the East. After Justinian's death, the Sassanid kings took advantage of Byzantium's internal weaknesses to launch a series of wars. The prolonged conflict with the Sassanids strained the Byzantine Empire's resources and led to significant military and territorial losses.
4. Slavs: The Slavic peoples began migrating into the Balkans during the 6th and 7th centuries. They settled in large numbers in the regions of the Balkans that had been under Byzantine control, challenging Byzantine authority and disrupting the empire's administration in these areas.
5. Arabs (Muslims): The rise of Islam in the Arabian Peninsula during the 7th century marked a significant threat to Byzantine power. The Arab conquests led by the Muslim Rashidun Caliphate rapidly expanded into the Byzantine territories in the Middle East, North Africa, and the Iberian Peninsula. The Byzantines suffered severe military defeats and territorial losses to the Arabs, which weakened the empire's position in these regions.
These various groups, along with internal political instability, economic challenges, and religious conflicts, posed significant military, political, and territorial threats to the Byzantine Empire in the period following Justinian's death. The empire struggled to maintain its control and territories in the face of these challenges, which shaped the course of Byzantine history in the centuries that followed.