There wasn’t a single cause. The fall of the Roman Empire—especially the Western Roman Empire in 476 CE—came from a mix of long-term problems:

- **Political instability**: Frequent changes of emperor, civil wars, and weak leadership made the empire hard to govern.
- **Economic troubles**: Heavy taxes, inflation, reliance on slave labor, and declining trade weakened the economy.
- **Military problems**: The army became expensive to maintain, less loyal to Rome, and increasingly dependent on foreign mercenaries.
- **Barbarian invasions**: Groups like the Visigoths, Vandals, and Ostrogoths entered and attacked Roman territory.
- **Overexpansion**: The empire was so large that it became difficult to defend and administer effectively.
- **Division of the empire**: Splitting into Eastern and Western halves made the West weaker, while the East survived as the Byzantine Empire.
- **Social decline**: Civic loyalty and public institutions weakened over time.
- **External pressure**: The Huns pushed Germanic peoples into Roman lands, increasing instability.

So, the Western Roman Empire fell because internal weakness and external invasions combined over time. The **Eastern Roman Empire** did not fall then—it continued for nearly 1,000 more years.
