Okay, so, just from the top of my head, I think that it is because of the growing belief of "Manifest Destiny" in the nineteenth century. I'm not sure if this is the actual right answer for your question, but I do know that Manifest Destiny was in the nineteenth century and was the widely held belief in the USA that American settlers were destined to expand through out the continent. And I think both the USA and Europe wanted to expand, it was kind of a copetition/conflict they had. This was actually how Canada and Alaska came to be I believe. This was a simple explanation btw. cx
Each of these events and actions dealt with the extension of slavery into the western territories. The Missouri Compromise (1820) banned slavery 36 degrees latitude. The Compromise of 1850 allowed for popular sovereignty (voters decide) in the Mexican Cession territory.
Based on the other answer, I'm guessing that it was A. Russians were likely to reject pure Marxism because Marx was a German. (I'll update it when I know for sure.)
