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The Russian Decembrists were sort of enamoured with Western political liberty and for many of them (like Prince Volkonsky) America was the shining example in this regard.

In a similar vein, after the failure of the 1848 German revolutions many middle-class Germans emigrated to the US, where they played a major part in contributing to their new country - especially with regards to the slavery question, which almost all were opposed to.

So the question is this: how would the US change if the Russian Decembrists (and their middle-class supporters) immigrated en masse to the United States in 1826? How would the politics, culture and society of the new country change with the addition of a new class that grew up with autocracy, serfdom and, also importantly, a strong French [not a typo] background?

For the sake of simplification, Let's say Tsar Nicholas allows this (instead of Siberia) and most of the Princes etc. bring some portion of their substantial wealth along, and the US accepts them. Obviously due to the nature of the Russian middle class the immigration wave isn't going to be as large, but then again the US isn't as populated back in the 1820s and the major Irish/German/Scandinavian waves haven't occurred yet, which gives the Russians some time to entrench themselves.
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