I predict that both of the two opposed errors of pessimism which now make so much noise in the world will be proved wrong in our own time‑the pessimism of the revolutionaries who think that things are so bad that nothing can save us but violent change, and the pessimism of the reactionaries who consider the balance of our economic and social life so precarious that we must risk no experiments.Brad DeLong explains how Keynesian policies helped prove their pessimism wrong:
The end in the Great Depression of laissez faire--the idea that the government should keep its hands off of the economy--as a doctrine for guiding economic policy did not mean the end of the market economy as a social resource allocation mechanism. "Keynesianism" and the doctrine of the "mixed economy" that it supported emerged in the nick of time, soon became the ruling ideologies in the industrial core of the world economy, and provided North America and western Europe with a Keynesian escape route from what had seemed the insoluble crises of the interwar period.
The Keynesian escape route opened up key ground in the middle between fascist-style regimentation and socialist-style national planning. Keynes argued that the market economy and capitalist order could be salvaged, and salvaged by relatively minor reforms. An activist welfare-state government with a commitment to full employment had the tools to eliminate Great Depressions, and could put economies back onto the road to Utopia. If only governments would reduce interest rates to get private agents or would themselves spend money freely (without raising taxes) in times when total demand was low, and raise interest rates to reduce private spending and themselves raise taxes (without raising spending) in times when total demand was high, then fiuctuations in employment and production could be greatly reduced, and Great Depressions avoided.
Belief in this escape route was strongly reinforced by facts. Those countries that had tried it by accident during the Depression--had infiated early, printed money, ensured low interest rates, and run large budget deficits--managed to survive the Depression much more easily than others. World War II provided final proof, were any necessary--"vindication by Mars," as John Kenneth Galbraith calls it. That component of unemployment, called "structural" or "permanent" during the 1930s, that was seemingly-immune to both the self-adjusting forces of the market and the armament of the New Deal vanished entirely in the 1940s as the federal budget deficit approached and then exceeded the levels that had long been recommended by John Maynard Keynes. And the United States fought World War II without reducing civilian consumption: all of U.S. war production came from new capacity or from capacity that stood idle at the end of the 1930s.
Demand expansion--deliberate attempts by governments to put the unemployed back to work by deficit spending and loose-money low interest rate policies--was successful in the 1930s and 1940s. It put the unemployed back to work. It did not contain within itself the seeds of a renewed Great Depression. It did not explode into hyperinflation. The coming of "stablization policy" enlarged the policy steps that could be undertaken without forcing a definitive break with the market-capitalist order, and without forcing a choice between Hitler's way and Stalin's.