[inglor_cz]

"What can't be ignored is the level of racism groups are subjected to: it seems obvious to say that people will integrate better if they have people willing to integrate with them. I suspect this is far more often the problem."

It does not seem as obvious to me. This theory seems to have too many outliers.

At least here in Central Europe, racism/anti-semitism against visible minorities tended to be very strong. Historically, Jews were treated horribly, and in the more recent history, Vietnamese also. Growing up in the 1990s, there was a lot of shockingly casual racism against them.

Looking across the pond, Japanese-Americans were herded into concentration camps during WWII. The only worse level of racism is probably genocide.

In all these cases, the communities are now very well integrated, though the memories of really bad treatment aren't that distant.