The purpose of this study is to examine a wide range of economic and military factors from the Third-Century Crisis to the fall of the Western Empire. My investigation of the Roman Economy focuses on taxation, inflation, coinage, mining, slavery and coloni. In the section on the Roman Army, I address policy decisions of emperors, logistics, pay and supply of the armies, military losses and recruitment, strategies and tactics, and the overall decline in population across the empire. By taking this broad approach and examining numerous factors, I will present a more complete explanation for the decline and eventual collapse of the Western Roman Empire. This explanation will show how the slow erosion of both the Roman economy and military from the third-century onward fatally weakened the Western Roman Empire causing it to fall apart.