1804...Under order of President Thomas Jefferson, the Lewis & Clark Expedition departs from St. Louis, Missouri, on May 14, to chart a course to the Pacific Ocean.
1841...First emigrant wagon train for California. Forty-seven people leave Independence Missouri, on May 1 and reach California on November 4.
1842...Settlement of Oregon begins via the Oregon Trail.
1847...After violent clashes with settlers over polygamy, Mormons leave Nauvoo, Illinois, and head for the west under Brigham Young. They eventually settle at Salt Lake City, Utah.
1848...Gold discovered January 24 in California. In February, Mexico ceded claims to Texas, California, Arizona, New Mexico, Nevada, Utah, and portions of Colorado.
U.S. pays Mexico $15 million.
1849-1850...80,000 prospectors immigrate to California to follow the gold boom...