https://www.theatlantic.com/business/archive/2017/02/scheidel-great-leveler-inequality-violence/517164/
https://politics.slashdot.org/story/17/02/21/2114240/the-only-thing-historically-thats-curbed-inequality-catastrophe
https://en.wikipedia.org/wiki/Great_Compression

The Atlantic has an interesting article on how societies have
decreased economic equality. From the report: "Calls to make America
great again hark back to a time when income inequality receded even as
the economy boomed and the middle class expanded. Yet it is all too
easy to forget just how deeply this newfound equality was rooted in
the cataclysm of the world wars. The pressures of total war became a
uniquely powerful catalyst of equalizing reform, spurring
unionization, extensions of voting rights, and the creation of the
welfare state. During and after wartime, aggressive government
intervention in the private sector and disruptions to capital holdings
wiped out upper-class wealth and funneled resources to workers; even
in countries that escaped physical devastation and crippling
inflation, marginal tax rates surged upward. Concentrated for the most
part between 1914 and 1945, this 'Great Compression' (as economists
call it) of inequality took several more decades to fully run its
course across the developed world until the 1970s and 1980s, when it
stalled and began to go into reverse. This equalizing was a rare
outcome in modern times but by no means unique over the long run of
history. Inequality has been written into the DNA of civilization ever
since humans first settled down to farm the land. Throughout history,
only massive, violent shocks that upended the established order proved
powerful enough to flatten disparities in income and wealth. They
appeared in four different guises: mass-mobilization warfare, violent
and transformative revolutions, state collapse, and catastrophic
epidemics. Hundreds of millions perished in their wake, and by the
time these crises had passed, the gap between rich and poor had
shrunk."