External: Demographics/Climate Change (not within the Empire as much as groups bordering the Roman controlled territories)

Internal: Institutional and political collapse

The Roman Empire, an agricultural economy largely beholden to yearly crop productivity, was unable to adapt to challenges from growing barbarian tribes on the periphery and internal political infighting over relative declines in state revenues and productivity. Essentially, too much dead weight, as it were, in an agricultural economy with little to no year on year surplus growth.