American believed they had a God-given mission to extend their system of democracy. American hoped to establish a democratic republic that reached to the Pacific
Many Americas believed their future prosperity depended on the land and wealth to be gained from westward expansion. The success of farmers in Texas and the discovery of gold in California encouraged this belief.
The pioneer spirit was seen as the perfect expression of American individualism. Americans admired the rugged pioneers who advanced into Texas, Oregon, California and the southwest.
Farmers were interested in expanding into the west to farm. Many farmers used slaves, so slavery was expected to expand into the west. The use of slavery caused tension between the north and the south.