What were the key factors leading to the collapse of the Roman Empire?

 My answer in Quora.

There were a number of factors (not just one). These include the devaluation of the currency, the outsourcing of security to third party mercenaries (barbarian troops who extracted large sums of money and land payments for their services), migration of foreign tribes westward into the Empire (Visigoths, Huns, Franks, Vandals etc), a falling work ethic, poor leadership, the splitting of the Empire into the wealthy East and the poor West, internal strife, the dilution of the notion of the responsible citizen, and conflicting ideological/theological struggles as Christianity replaced paganism.

It is worth noting that the Western Empire collapsed almost a millennium before that of the East Byzantine Empire (476 vs 1453 AD). Many of these factors listed above are more relevant to the West. The East was weakened by the Arab invasion, conflicts with the Crusaders and ultimately the rise of first the Seljuks then Ottoman Turks. There was also considerable infighting between the various Byzantine royals.

If you see any of these analogous trends in today’s contemporary world, Be concerned. Very concerned.