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Prior to World War II American foreign policy was isolationist. We felt that other nations problems, particularly their wars, were their own business and we avoided getting involved unless we felt directly threatened. As a result of WWII though we decided that threats to peace and freedom elsewhere in the world did affect us, that if we ignored serious trouble in the world it would probably eventually find us. Thus after the war we became internationalist using our power and prestige to help and protect our friends and acting to prevent wars wherever possible or to minimize them when they did break out.

Another issue that drove foreign policy post WWII was the spread of communism from both the Soviets and Chinese. No longer could the US afford to be isolationist. The African continent saw decolonization and by the 1960s the fight was under way for countries between democratization and Marxism. There fore the US supported autocracies and not "freedom fighters" generally aligned with Marxist regimes.
The Keynesian explanation for the Great Depression came under came under heavy fire in 1963, when Milton Friedman and Anna Schwartz published A Monetary History of the United States. Free-market economists philosophically opposed to the heavy government interventionism unleashed by Keynesianism, Friedman and Schwartz made a compelling argument that the Great Depression had been caused less by a failure of aggregate demand than by a sharp constriction in the nation's money supply. Foolish decisions by the Federal Reserve, they argued, combined with hoarding of cash by individuals fearful of bank failures, caused the stock of money circulating in the economy to fall by one-third between 1929 and 1933. This "Great Contraction," as Friedman called it, had a choking effect on employment, incomes, and prices, unnecessarily prolonging the Great Depression by years. The New Deal's Keynesian intrusion into the free market had done little to address the underlying money