- Barbarian Invasions: During Justinian's reign, the empire faced numerous invasions from various barbarian groups, including the Lombards, Avars, and Bulgars. These invasions put a significant strain on the empire's resources and military strength.
- Financial Exhaustion: Justinian's extensive military campaigns, including the reconquest of North Africa and Italy, placed a heavy financial burden on the empire. The high costs of maintaining large armies and conducting wars drained the empire's treasury and led to increased taxation, causing discontent among the population.
- Overextended Borders: The Byzantine Empire under Justinian reached its greatest territorial extent, encompassing vast regions in Europe, North Africa, and the Middle East. However, this vast expanse made it difficult to effectively defend and control the frontiers. The empire's long supply lines and dispersed troops left them vulnerable to enemy attacks and rebellions.
- Religious Conflict: Religious differences and theological disputes posed significant challenges to the empire's unity. The Monophysite controversy, which centered around the nature of Christ, caused divisions within the empire and alienated many Christian communities in Egypt, Syria, and Armenia, weakening the empire's religious and political cohesion.
- Plague (Justinian's Plague): The bubonic plague, known as the Justinian's Plague, struck the empire in the mid-6th century. It caused widespread devastation, killing millions and disrupting trade, agriculture, and urban life. The plague severely weakened the empire's economy, military strength, and overall stability, making it more challenging to maintain and control its territories.
- Imperial Overextension: Justinian's ambitious military campaigns and conquests required a massive and expensive military apparatus. Maintaining such a large army placed significant strain on the empire's economy, leading to fiscal challenges and reducing the resources available for other sectors, such as infrastructure, education, and societal development.
- Loss of Territories: Justinian's conquests were short-lived. Many of the territories acquired during his reign, including parts of Italy and North Africa, were eventually lost to rival powers and barbarian invasions in subsequent decades, further shrinking the empire's borders and diminishing its strength.
- Rise of Rival Powers: The emergence and expansion of rival powers around the Byzantine Empire, such as the Sassanid Persians in the East and the Visigoths in Spain, challenged Byzantine dominance and further stretched its military and diplomatic resources, making it more difficult to hold onto conquered territories.