There are several reasons for the fall of the Western Roman Empire, but one commonly cited reason is the invasion by barbarian tribes. These tribes, including the Visigoths, Huns, and Vandals, began to attack the empire's borders in the late 4th century CE and eventually overran the western half of the empire, leading to its collapse in 476 CE. The barbarian invasions weakened the military and political power of the empire, making it difficult for the Roman army to defend against the constant attacks. Additionally, the empire faced economic problems, social unrest, and political instability, which further contributed to its decline and eventual collapse.