Which factors led to the mid-1800’s westward expansion in the united states?

I would say that European immigration to the US plus the fact that it was no longer a colony would probably attract people plus the west had large unoccupied spaces including the Great Plains (apart from the First Nations who had lived there for eons) and I believe the US government granted land concessions to attract settlement and agriculture in the west and also many tribes had been forced onto reservations so settlement was met with less native opposition.

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It is the doctrine that the us had the right and duty to expand throughout north america. it just means they believed they had the right to expand westward and they did! they stated with the 13 colonies and then they kept moving gaining different territories throughout the years