The fall of the Roman Empire, particularly its Western division, was the result of a combination of internal and external factors. Here are some of the main reasons:

1. **Economic Decline**: The economy of the Roman Empire faced significant challenges, including inflation, currency devaluation, and the over-reliance on slave labor, which reduced productivity and innovation. Additionally, heavy taxation to support military spending further burdened the populace.

2. **Military Overstretch**: The vast size of the empire made it difficult to defend all borders effectively. The constant need for military campaigns drained resources and weakened the central authority. Furthermore, reliance on mercenary troops and less-disciplined barbarian auxiliaries undermined the traditional Roman army's strength.

3. **Barbarian Invasions**: External pressures from various "barbarian" groups such as Goths, Vandals, Huns, and others increased. These groups crossed the frontiers, often seeking refuge or land, and eventually became a direct threat to Roman control. Notable events include the sack of Rome by the Visigoths in 410 AD and by the Vandals in 455 AD.

4. **Political Instability**: The Roman Empire experienced frequent civil wars, power struggles, and weak emperors, leading to political instability. The division of the empire into Eastern and Western halves in 285 AD (later formalized in 395 AD) also contributed to this fragmentation. The Western Roman Empire, in particular, saw a rapid succession of emperors, many of whom were appointed by the military rather than through legitimate means.

5. **Social Unrest and Corruption**: Social inequality, widespread poverty, and corruption within the government and military eroded public trust and cohesion. The elite class often lived in luxury while the common people suffered, leading to discontent and social unrest.

6. **Cultural and Religious Changes**: The rise of Christianity and its eventual adoption as the state religion under Emperor Constantine changed the cultural landscape. While this shift unified some aspects of society, it also led to conflicts with traditional Roman paganism and sometimes contributed to divisions within the empire.

7. **Environmental Factors**: Some historians suggest that environmental changes, such as climate change and crop failures, may have played a role in weakening the empire's agricultural base and contributing to economic hardships.

These factors, acting together over several centuries, ultimately led to the collapse of the Western Roman Empire in 476 AD, when the last Roman emperor, Romulus Augustulus, was deposed by the Germanic chieftain Odoacer. The Eastern Roman (Byzantine) Empire continued for nearly a thousand more years.