### Key reasons (mainly for the Western Empire’s fall in 476 CE)
- **Political instability**: Rapid turnover of emperors, coups, succession crises.
- **Civil wars**: Internal conflicts drained resources and fractured authority.
- **Military overextension**: Vast, costly frontiers that were hard to defend.
- **Barbarian pressures**: Goths, Vandals, Huns; sacks of Rome (410, 455); Odoacer in 476.
- **Economic weakness**: Heavy taxation, currency debasement, inflation, trade disruption.
- **Shrinking tax base**: Ruralization, large estates, urban decline, labor shortages.
- **Reliance on federates/mercenaries**: Troops with mixed loyalties and political leverage.
- **Administrative corruption/inefficiency**: Bloated bureaucracy, graft, poor governance.
- **Plagues and demographic shocks**: Population losses reduced manpower and revenue.
- **Environmental/climate stress**: Agricultural shortfalls from droughts/cold spells.
- **Strategic East–West split**: Wealth and stability concentrated in the East (Byzantium).
- **Cultural shifts (debated)**: Changing civic norms and the Church’s rising role altered state priorities.