Short answers, pulled from the story.
John Maynard Keynes published The General Theory of Employment Interest and Money in 1936 during the depths of the Great Depression. This work argued that periods of high unemployment could persist indefinitely without government intervention.
Richard Kahn introduced a ratio describing how initial public spending generated secondary rounds of consumption. He envisioned money passing from hand to hand creating employment at each step until it reached a cul-de-sac through imports or hoarding.
Stagflation meant rising unemployment coupled with rising inflation simultaneously during the 1970s. This phenomenon contradicted Phillips curve predictions about trade-offs between inflation and unemployment and caused traditional models to fail.
Western industrialized countries adopted Keynesian principles between 1945 and 1973 during what historians call the Golden Age of Capitalism. Governments prepared high quality economic statistics on an ongoing basis to guide policy decisions while most nations enjoyed low stable unemployment and modest inflation throughout this period.
Douglas Greenwald and Joseph Stiglitz represent New Keynesian economists showing how market failures regarding credit rationing lead to persistent unemployment. They demonstrate wage rigidity can cause modern economies to struggle even when demand exists.