Historians don’t agree on a single “cause”; the Western Roman Empire fell because several long‑term internal weaknesses combined with short‑term shocks. Key factors:

1. **Political instability and civil wars**  
   - Emperors were frequently overthrown or murdered, especially in the 3rd–5th centuries.  
   - Competing generals (usurpers) raised their own armies and fought for the throne.  
   - Constant civil wars drained money and troops that were needed on the frontiers.

2. **Military problems and reliance on “barbarian” troops**  
   - The army became harder to recruit and more expensive to maintain.  
   - Rome increasingly relied on Germanic federate troops (Foederati) led by their own chiefs.  
   - Loyalty shifted from the Roman state to individual generals and barbarian leaders.  
   - Catastrophic defeats, like Adrianople (378 CE), weakened Rome’s ability to control its borders.

3. **Barbarian migrations and invasions**  
   - Pressure from the Huns and others pushed Germanic peoples into Roman territory.  
   - Visigoths, Vandals, Ostrogoths, Franks and others settled inside the empire, sometimes as allies, sometimes as invaders.  
   - Rome was sacked twice in the 5th century: by the Visigoths (410) and the Vandals (455).  
   - In 476, the Germanic leader Odoacer deposed the last Western emperor, Romulus Augustulus.

4. **Economic decline and fiscal crisis**  
   - Heavy taxation to support the army and bureaucracy burdened farmers and urban populations.  
   - Coinage was debased over time, contributing to inflation and loss of confidence in money.  
   - Trade became less secure as piracy and banditry increased, especially in the late empire.  
   - Large estates (latifundia) and a semi‑free peasant class (coloni) undermined small farmers and civic life.

5. **Demographic and social strain**  
   - Plagues (e.g., Antonine Plague, Cyprian Plague) reduced population, shrinking the tax base and pool of recruits.  
   - Urban decline and a rigid social hierarchy reduced social mobility and local initiative.  
   - Many people saw the imperial system as distant and oppressive, weakening civic loyalty.

6. **Administrative overextension and division of the empire**  
   - The empire was vast and diverse; governing it from a single center was difficult.  
   - Diocletian split the empire into eastern and western halves for administrative reasons.  
   - The richer, more urbanized Eastern Empire (later “Byzantine”) could better survive crises; the poorer Western Empire struggled to defend itself.

7. **Religious and cultural change (contested factor)**  
   - The spread of Christianity changed the ideological basis of the empire, shifting emphasis from traditional Roman civic religion to a universal faith.  
   - Some older historians (e.g., Gibbon) blamed Christianity for weakening Roman martial values, but most modern scholars see it as one factor among many, not a primary cause.

8. **Environmental and external pressures (possible contributors)**  
   - Climate fluctuations may have affected agriculture and migration patterns.  
   - Steppe nomads (Huns and later others) created cascading movements of peoples into Roman territories.

**Why the East survived while the West fell**  
- The Eastern Empire had richer provinces, stronger cities, better fortifications, and more stable tax revenues.  
- It could pay off or redirect invaders more effectively and maintain a stronger professional army.  
- The Western Empire, with weaker finances and more exposed frontiers, collapsed under combined internal weaknesses and external pressures, while the East transformed into the Byzantine Empire and continued for nearly 1,000 more years.

In short, the fall of the Western Roman Empire (traditionally dated 476 CE) was the outcome of long‑term internal problems—political, military, economic, and social—interacting with intense external pressures from migrating and invading peoples.
