According to Walter Scheidel, author of The Great Leveler, great inequality in societies is only reduced by war, plagues, natural disasters, or communism. There is no historical precedent for a peaceful political resolution to radical inequality. Over time great inequality causes political instability. This leads to internal contradictions within the system that are unresolvable without extreme measures.
The Roman economy was based on continued expansion. It would also enslave the defeated people. Slavery reduced the incentives for technological innovation. The problem is that you cannot expand forever. Geography has its limits.
1. Invasions by Barbarian tribes
The most straightforward theory for Western Rome’s collapse pins the fall on a string of military losses sustained against outside forces. Rome had tangled with Germanic tribes for centuries, but by the 300s “barbarian” groups like the Goths had encroached beyond the Empire’s borders. The Romans weathered a Germanic uprising in the late fourth century, but in 410 the Visigoth King Alaric successfully sacked the city of Rome. The Empire spent the next several decades under constant threat before “the Eternal City” was raided again in 455, this time by the Vandals. Finally, in 476, the Germanic leader Odoacer staged a revolt and deposed the Emperor Romulus Augustulus. From then on, no Roman emperor would ever again rule from a post in Italy, leading many to cite 476 as the year the Western Empire suffered its deathblow. …