1929, the great depression kicks off, 1933 the new deal is enacted, government and stimulus and deficits help the economy start to recover, 1936 Keynes & Co advocate government deficit spending to help bounce back from recessions, policy makers don’t listen and in 1937 FDR actually starts to cut back on government spending resulting in another slump. Then in 1939 World War II kicks off and the government massively increases spending in preparation for and then on the war and Keynesian theory gets it’s ultimate validation as the economy rapidly returns to full employment. I wouldn’t exactly call World War II a ‘happy little accident’ in this respect but it is interesting that so soon after Keynes theory was published it got such a massive experiment-test-case whose results showed it to be pretty on the mark.