There are several primary factors that contributed to the decline of the Roman Empire:

1. Economic issues: The empire faced high inflation, heavy taxation, and a devaluation of currency, which weakened the economy.

2. Military challenges: The Roman army was stretched thin defending the empire's vast borders. Additionally, the increasing reliance on foreign mercenaries reduced the army's loyalty and effectiveness.

3. Political instability: The Roman Empire faced frequent power struggles, assassinations, and a rapid succession of emperors, leading to a lack of stable leadership.

4. Moral and societal decay: Many historians argue that a decline in civic responsibility, increasing corruption, and a growing gap between the rich and poor contributed to the empire's decline.

5. Barbarian invasions: The empire faced repeated invasions by various barbarian tribes, such as the Goths, Vandals, and Huns, which gradually weakened its military and economic power.

6. Overexpansion: As the Roman Empire grew, it became increasingly difficult to manage and defend, with the vast territory and diverse populations posing significant logistical challenges.

7. The rise of Christianity: Some scholars argue that the spread of Christianity, which emphasized spiritual rewards over worldly concerns, may have contributed to a decline in civic engagement and military readiness.

These factors combined over time gradually weakened the Roman Empire, eventually leading to its fall in 476 AD when the Germanic chieftain Odoacer deposed the last emperor of the Western Roman Empire.