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.
Americans believed that they had a God-given mission to expand their system of democracy. Americans hoped to establish a domestic republic that reached to the Pacific, which would serve as an example to the world.
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.
The pioneer spirits was seen as the perfect expression of American individualism. Americans admired the rugged pioneers who advanced into Texas, Oregon, California, and the Southwest.
There was a desire for American businesses to open up more markets to sell their goods to other countries. This could be done in the West, ultimately with in the North, South America and Canada.