The decline of the Roman Empire (especially the Western Empire, which fell in 476 CE) resulted from interconnected factors spanning centuries. Modern historians emphasize **multicausality**, but key elements include:

1. **Military Challenges**:  
   - **Overextension**: Defending vast borders from Germanic tribes, Persians, and others strained resources.  
   - **Mercenary Reliance**: Hiring foreign mercenaries (foederati) eroded army discipline and loyalty.  
   - **Defeat in Key Battles**: Losses like the Battle of Adrianople (378 CE) depleted military capacity.

2. **Political Instability**:  
   - **Leadership Crisis**: Frequent assassinations, civil wars (e.g., the Year of the Five Emperors, 193 CE) and short-lived rulers weakened governance.  
   - **Division of the Empire**: The East-West split (285 CE) divided resources, making coordinated defense harder.

3. **Economic Troubles**:  
   - **Debasement of Currency**: Inflation soared as silver content in coins decreased.  
   - **Tax Evasion**: Wealthy elites avoided taxes, shifting burdens to the lower classes.  
   - **Trade Disruption**: Insecure trade routes reduced revenue and access to goods.

4. **Social and Cultural Shifts**:  
   - **Labor Shortages**: Reliance on slave labor declined, coupled with plagues (e.g., Antonine Plague, 165‒180 CE).  
   - **Christianity's Rise**: Traditional civic values eroded, though Christianity also provided cohesion later in the East (Byzantine Empire).  

5. **Environmental Pressures**:  
   - **Agricultural Decline**: Soil exhaustion and deforestation reduced food production.  
   - **Climate Change**: The "Late Antique Little Ice Age" (c. 536‒660 CE) caused crop failures and famine.  

6. **External Invasions**:  
   - **Migratory Pressures**: Goths, Vandals, Huns, and others overwhelmed frontiers after crossing the Rhine/Danube (e.g., 406 CE "Crossing of the Rhine").  
   - **Sack of Rome**: Alaric's Visigoths (410 CE) and the Vandals (455 CE) accelerated symbolic decline.  

### Scholarly Perspectives  
- **Edward Gibbon** (*Decline and Fall*, 1776) emphasized moral decay and Christianity’s impact (less accepted today).  
- **Modern Views**: Focus on systemic issues like logistical overstretch, economic fragility, and cascading failures.  

The **Eastern Empire survived as Byzantium** until 1453 due to stronger defenses, stable revenue, and strategic adaptability, underscoring the West’s unique vulnerabilities.  

For deeper analysis: Peter Heather's *The Fall of the Roman Empire* (2005) or Bryan Ward-Perkins' *The Fall of Rome* (2005) offer comprehensive interdisciplinary insights.