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What is it about Europe that has made it so welcoming for American jazz musicians? *****
I would say France rather than Europe. The French never miss an opportunity to show the 'superiority' of their country, Language and culture. It still goes on today. They see them selves as a counter-weight to American influence and power in the world.
The expats: These were people like Monk, Bird, Richard Wright, James Balwin, Josephine Baker and other great musicians, writers and entertainers. What's not to like. Of course they were welcomed by the French elite, located mainly in Paris.
They did not welcome black people, they welcomed great artists who happened to be black, something new and different, and very few in number.
What the expats should have known was that if the entire US black population had moved to France, the reception would have been very different, as it is today, after North Africa showed up in France.
BTW, the black female Jazz trumpet player, Valaida Snow happened to be touring in Denmark when Hitler declared war on the USA. She was promptly arrested and thrown in prison as an enemy alien. Just saying. She was later part of a prisoner exchange.
Cheers