Can you elaborate this? How does the War of 1812 ended mixed marriages in the US? I'm curious.
By "mixed marriages", I was talking about marriages between whites and natives, not necessarily whites and blacks.
IIRC, the War of 1812 removed any possibility of Americans viewing Native Americans as equals, mainly because they sided with the British. It was also the first time that American military forces were segregated. In "Lies My Teacher Told Me", John Loewens writes that racially integrated societies and mixed marriages were common in the Old Northwest prior to the War of 1812 (mainly between Whites and Natives).
To deal with anti-Asian racism and increase the percentage of mixed-race Americans:
-Increase Chinese immigration (no Chinese Exclusion Act). Hopefully, they'll integrate the way the Irish and Italian did.
-Keep the Philippines as part of the US (although this is really difficult)
-No war with Japan couldn't hurt either.
As for anti-Hispanic racism, I think the key problem here is Catholicism, which many early 19th-century Americans tolerated about as much as paganism. If Americans can accept Catholicism, then they're much more likely to view the white latinos as equals. Maybe Quebec joining the union? (But good luck getting that to happen)
Edit: Reading back over my response, it seems like there is no problem that can't be solved by increasing the population and territory of the United States.
