The key factor that led to the end of the Roman Empire was a combination of internal strife and external pressures. Internally, political instability, economic decline, and social decay weakened the Empire's structure. Externally, invasions by various barbarian groups, such as the Visigoths and Vandals, created military challenges that the Empire struggled to address. The culmination of these issues led to the fall of the Western Roman Empire in 476 AD, when the last emperor, Romulus Augustulus, was deposed. This collapse marked the transition to the Middle Ages and drastically altered European history.