The decline of the Roman Empire, which spanned both the Western and Eastern (Byzantine) parts, was a complex process influenced by a combination of internal and external factors. Here are some of the primary contributing factors:

1. **Political Instability**: Frequent power struggles, civil wars, and the assassination of emperors led to political instability. The empire saw many rulers come and go, often leading to weak governance and a lack of long-term vision.

2. **Economic Troubles**: Economic challenges included high taxes, inflation, and a declining agricultural base. The cost of maintaining the military and bureaucracy became unsustainable, while the tax system became increasingly burdensome on the populace.

3. **Military Overstretch**: The vast size of the empire made it difficult to defend effectively. The constant need for military expansion and defense against external threats drained resources. Additionally, the reliance on mercenaries weakened the loyalty and effectiveness of the Roman legions.

4. **Barbarian Invasions**: Continuous invasions and pressure from various barbarian groups such as Goths, Vandals, Huns, and others were significant external pressures. These invasions eventually led to the sacking of Rome in 410 AD and 455 AD, severely weakening the empire.

5. **Social Changes**: Social issues such as the growing gap between rich and poor, and the erosion of traditional Roman values also played a role. The increasing influence of Christianity introduced new social dynamics and sometimes conflicted with traditional Roman practices.

6. **Environmental Factors**: Some historians suggest that environmental changes, including climate change and natural disasters, may have contributed to the decline by affecting agriculture and food supplies.

7. **Administrative Difficulties**: As the empire expanded, the central administration struggled to manage its vast territories effectively. This decentralization led to regional disparities and a dilution of imperial authority.

8. **Division of the Empire**: The division of the Roman Empire into the Western and Eastern halves under Emperor Diocletian in the late 3rd century further weakened the Western Roman Empire, which eventually fell in 476 AD. The Eastern Roman (Byzantine) Empire continued to thrive for nearly another millennium.

Each of these factors, acting alone or in combination, contributed to the gradual decline and eventual fall of the Western Roman Empire, while the Eastern Roman Empire survived for much longer due to its relative isolation and fewer external threats.