I wonder if Jews are so widely persecuted because they are in so many places?

They are the other co-existing culture in so very many places. So many opportunities for persecution. If you go to a random spot on the planet and punch the first person who doesn't look like the majority in that spot, chances are you punched a Jew.

To put it another way, the Nazis didn't persecute Native Americans. It wasn't because they liked them.

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As to co-existence being not a supportable option, are there any single-culture nations that accomplish anything? Prime example: Spain with Jews - world power; Spain post 1492 (when they evicted the Jews) - punch line. Costa Rica, the only Latin American country with no Indians (they vanished completely before the Europeans arrived) is a beautiful, peaceful land. You would not believe the place, just stunning, every environment except arctic within a day's drive, more than half the land (I was told) is National Park. And actual trained grown-up cops with side-arms instead of army boys with machine guns like most of Central America. But isn't really doing anything at all. The United States, probably the most co-existence-based nation ever, not to be all "we're number one" but we are one hell of a vibrant, productive, interesting country. We landed on the goddamn Moon, ya know? Everybody goes to our movies, buys our crap or at least Chinese crap with our brands.

Co-existence is messy, but the alternative seems to me to be stagnation.

Hell, I bet Great Britain would never have been important if the Angles and Saxons had been able to get along.