The correct option is: "d. all of the above"
At the beginning of the 19th century, the West was considered a wild and inhospitable territory, with few possibilities of being inhabited. This opinion was not influenced by the fact that the Amerindian peoples had lived there for several millennia. The advance of the "civilized East" over the "wild West" was the encounter and clash between two mutually exclusive worlds. The expansion of the US border was possible thanks to a series of political, civil and military events that occurred during the 19th century. Some had government support. Others, on the contrary, were born of individual initiative.