The idea of manifest destiny.

Manifest destiny is belief held by many Americans in the 1840s that the United States was destined to expand across the continent, by force, as used against Native Americans, if necessary. The controversy over slavery further fueled expansionism, as the North and South each wanted the nation to admit new states that supported its section's economic, political, and slave policies. By the end of the 19th century, this belief was used to support expansion in …

…because it did not have the resources to meet it head on. I think that this expansionism was very obvious by the late 1830s and most of all by the early '40s. In the early '40s, countless books appeared on the subject of California emphasizing its richness, its mild climate, its beautiful and marvelous ports, which could be used in this Utopia that the United States had always had about trading with China.