In The Great Leveler, Walter Scheidel argues that over thousands of years, only mass violence and catastrophes have triggered significant reductions in inequality.
From the book’s introduction:
For thousands of years, civilization did not lend itself to peaceful equalization. … stability favored economic inequality. This was as true of Pharaonic Egypt as it was of Victorian England, as true of the Roman Empire as of the United States. … Four different kinds of violent ruptures have flattened inequality: mass mobilization warfare, transformative revolution, state failure, and lethal pandemics.
… there is no compelling empirical evidence to support the view that modern economic development, as such, narrows inequalities. There is no repertoire of benign means of compression that has ever achieved results that are even remotely comparable to those produced by the Four Horsemen.