The primary factor in the decline of the Roman Empire was a combination of internal strife, economic instability, and external pressures from invading barbarian tribes. Political corruption, civil wars, and ineffective leadership weakened the central authority. Economic troubles, including high taxation and reliance on slave labor, reduced productivity and trade. Meanwhile, external attacks from groups like the Visigoths and Vandals exploited these weaknesses, culminating in the sack of Rome in 410 AD and the eventual fall of the Western Roman Empire in 476 AD, leading to a significant power shift in Europe.