Notes

The Louisiana Purchase, which took place in 1803, doubled the land area of the United States. The Corps of Discovery expedition led by Lewis and Clark gave the American government and people a glimpse of the lands to the west. Jefferson's concept of an "empire of liberty" for enterprising, independent people was carried on by succeeding presidents. A journalist named John O'Sullivan came up with the term "manifest destiny" to describe the moral obligation Americans had of spreading their institutions of liberty. This idea was formalized in 1823 with the Monroe Doctrine, which stated that the United States considered any further European colonization in the Western Hemisphere an act of aggression.
Although westward expansion created opportunities for settlers to acquire land, the question arose as to whether slavery would be allowed in the new states. Compromises ensued as Congress attempted to maintain a balance between free states and slave states, but the controversy eventually erupted into the Civil War. Additionally, as settlers spread westward, Native Americans were pushed off their lands and violently suppressed. When negotiations with Mexico for the purchase of parts of Texas, New Mexico, Nevada, California and other territories broke down, the Mexican-American war was fought. In 1848, the Treaty of Guadalupe that ended the war added an area larger than the Louisiana Purchase to the United States.