The spread of liberal ideas has been precipitated by more than 250 years of historical experiments. Economic thought can be said to have evolved since the father of modern economics, Adam Smith, published "An Inquiry into the Nature and Causes of the Wealth of Nations" in 1776.
Karl Marx then wrote his famous work, "Das Capital", in 1867. It was a book spurred on by his belief that working conditions for the proletariat class were far from acceptable, as compared to the bourgeoisie. In the previous century, before the emergence of capitalism, working conditions were probably worse.
More recently, John Maynard Keynes gained public prominence with "The Economic Consequences of the Peace", which correctly predicted that the reparations demanded by the Allies at the end of World War I would have disastrous consequences because Germany could not afford to pay them. Another book written by Keynes, "The General Theory of Employment, Interest and Money", published in 1936, argued that laissez-faire was an obsolete theory and economies left to their own devices could become trapped in depressions.
Keynes revolutionised the way the world thought about economics but, by the 1970s, the use of his policies to solve economic problems started to go out of fashion. People like Milton Friedman offered valid reasons for the emergence of stagflation (growing unemployment and high inflation) that plagued the 1970s, fuelling doubt about the efficacy of Keynes' policies.
The monetarists, led by Friedman, argued that Keynes' policies had led to larger budget deficits and inflation due to the excessive growth in money supply caused by the deficits. While Keynes saw the Great Depression as a consequence of the failure of markets, Friedman concluded that fault lay with government.
Joseph Schumpeter in his book "The Theory of Economic Development", wrote that a depression, even an extended one, is no reason to doubt capitalism.
Soon Friedman became the public face for a whole school of economic thinking, and what he advocated began to be keenly listened to by many world governments; his push for a restoration of market-driven forces to economic decision-making was well on its way.