The decline of the Roman Empire was a complex and multifaceted process that occurred over several centuries. Historians generally identify a combination of internal and external factors that contributed to this decline:

1. **Political Instability**: The Roman Empire faced significant internal strife, including civil wars, power struggles, and frequent changes in leadership. Emperors were often overthrown, leading to a lack of continuity and stable governance.

2. **Economic Problems**: The empire suffered from severe economic difficulties, including excessive taxation, inflation, and reliance on slave labor, which stifled innovation. The debasement of currency to support military expenses further weakened the economy.

3. **Military Issues**: The Roman military struggled with recruitment and discipline as Rome became increasingly dependent on mercenaries, many of whom had little loyalty to the empire. Frequent invasions by barbarian tribes, such as the Visigoths, Vandals, and Huns, put immense pressure on Roman defenses.

4. **Overexpansion**: The vast size of the empire made it difficult to govern and defend effectively. Resources were stretched thin, leading to vulnerabilities along the extensive borders.

5. **Social and Cultural Decay**: There was a decrease in civic pride and engagement among Roman citizens. The traditional Roman values and identities became diluted as the empire integrated diverse cultures and peoples.

6. **Administrative Inefficiency**: The empire's bureaucratic systems became increasingly cumbersome and corrupt. Effective governance was hampered by overlapping authorities and widespread corruption among officials.

7. **Religious and Ideological Changes**: The rise of Christianity and its adoption as the state religion radically transformed Roman society and weakened traditional Roman religious and cultural institutions.

8. **Environmental and Health Factors**: Possible environmental changes, such as soil depletion and deforestation, might have impacted agricultural productivity. Additionally, plagues and epidemics reduced the population, affecting labor availability and economic productivity.

These factors combined in various ways over time, leading to the gradual weakening of the Roman Empire and eventually its fall, traditionally marked by the sack of Rome in 410 AD and the abdication of the last Roman emperor in the West, Romulus Augustulus, in 476 AD.