The most severe global economic crisis of the 20th century — triggered by the Wall Street Crash of 1929, propagated by bank failures, trade collapse, and deflationary spirals, and ending only with World War II mobilization — which permanently changed how governments understood their responsibility for economic stability and generated both the New Deal and the political conditions for fascism.
At its worst, one in four Americans was unemployed. Banks closed by the thousands. Farm prices collapsed. The conventional wisdom said governments should do nothing — that the economy would correct itself. The Depression didn't just destroy livelihoods. It destroyed a theory of government. What came after — the welfare state, Keynesian economics, deposit insurance, financial regulation — was built on the ruins.