A great civilization is not conquered from without until it has destroyed itself within. The essential causes of Rome’s decline lay in her people, her morals, her class struggle, her failing trade, her bureaucratic despotism, her stifling taxes, her consuming wars.

William James (Will) Durant (1885-1981) American historian, teacher, philosopher

The Story of Civilization, Vol. 3: Caesar and Christ (1944)
    (Source)