Why Booms After Every War?
Actually, if one goes back in history, the World War II postwar boom was preceded not just by the Roaring Twenties, but by a historical pattern that people were noticing (the nuclear stalemate of the Cold War finally ended it).
An economic boom is ended by excessive speculation - the bubble collapses. People are left out of work, but the government isn't going to massively intervene in the economy to end the human misery - that sort of thing just isn't done.
And then a war starts. Well, that's national survival for you, so of course deficit financing is now permissible. And so what was impossible before becomes possible now - people are put to work.
The war ends. Now the economy is humming, but its products can be put towards the service of real human needs. An economic boom.
Keynes wasn't the only one to suggest getting out of this circle. Unrestrained free enterprise is not capable of producing an economy with no crashes ever and full employment always - that takes a hand at the switch ready to limit or remedy the potential damage of excessive speculation. As ordinary working men far outnumber businessmen, in a democracy it is their economic interests that should be jealously safeguarded by the government.