The fall of the Western Roman Empire (476 AD) was the result of a complex interplay of interconnected factors. Here are the main reasons, organized thematically:

### 1. **Political Instability and Weak Leadership**  
   - **Frequent Changes in Leadership**: The empire experienced a high turnover of emperors, often through assassination or civil war (e.g., the "Year of the Five Emperors" in 193 AD), leading to chronic instability.  
   - **Lack of Strong Succession Systems**: Inheritance of power was often chaotic, resulting in power struggles and civil wars.  
   - **Bureaucratic Corruption**: Overly complex and inefficient governance systems, coupled with corruption, eroded administrative effectiveness.  

### 2. **Economic Decline**  
   - **Inflation and Currency Debasement**: The 3rd-century crisis saw the devaluation of currency due to excessive minting of silver coins, leading to hyperinflation.  
   - **Heavy Taxation**: High taxation to fund the military and bureaucracy strained the economy, leading to unemployment and reduced productivity.  
   - **Reliance on Slave Labor**: Dependent on slavery, the economy stagnated, stifling innovation and technological advancement.  

### 3. **Military Challenges**  
   - **Overextension**: The vast empire was difficult to defend, requiring constant military resources.  
   - **Dependence on Mercenaries**: Hiring barbarian mercenaries led to loyalty issues and weakened military cohesion.  
   - **Internal Power Struggles**: The military often seized power, further destabilizing the state (e.g., 3rd-century military emperors).  

### 4. **Social and Cultural Factors**  
   - **Decline in Civic Virtue**: A loss of traditional Roman values (e.g., duty, discipline) and public engagement eroded social cohesion.  
   - **Class Divides**: Growing inequality between the elite and the lower classes fueled unrest and reduced popular support for the state.  
   - **Religious Shifts**: The adoption of Christianity as the state religion (after Constantine) shifted cultural priorities and, in some views, weakened the empire’s traditional martial ethos.  

### 5. **External Invasions and Pressure**  
   - **Barbarian Invasions**: Germanic tribes (e.g., Visigoths, Vandals, Ostrogoths) and other groups repeatedly attacked the empire, overwhelming its defenses.  
   - **Military Overstretch**: The empire struggled to repel invasions while maintaining internal order, especially after the division into East and West.  

### 6. **Division of the Empire**  
   - **Diocletian’s Reorganization (285 AD)**: Splitting the empire into Eastern and Western halves (with Constantinople as a new capital) shifted focus and resources to the East, leaving the West vulnerable.  
   - **Strained Resources**: The Western Empire bore the brunt of external threats and internal chaos with fewer resources.  

### 7. **Environmental and Demographic Pressures**  
   - **Climate Change**: Some theories suggest minor climate shifts (e.g., cooling periods) may have impacted agriculture, though this remains debated.  
   - **Plague and Population Decline**: Epidemics (e.g., the Antonine Plague) reduced population and labor, weakening the economy.  

### 8. **Loss of Territorial Control**  
   - **Germanic Kingdoms**: The Western Empire gradually lost control of provinces, with Germanic tribes establishing independent kingdoms.  
   - **Fall of the Last Emperor**: The deposition of Romulus Augustulus in 476 AD marked the end of the Western Empire, though the Eastern (Byzantine) Empire survived until 1453.  

### Summary  
The fall of the Western Roman Empire was not due to a single cause but a combination of political, economic, social, military, and external factors. While the Eastern Empire endured longer due to its stronger economy and strategic location, the Western Empire’s inability to adapt to these challenges led to its collapse. Historians continue to debate the relative importance of each factor, but these elements collectively contributed to the empire’s decline.