Major settlement of the western territories by migrants from the states in the east developed rapidly in the 1840s, largely[citation needed] through the Oregon Trail and the California Gold Rush of 1849; California experienced such a rapid growth in a few short months that it was admitted to statehood in 1850 without the normal transitory phase of becoming an official territory. The largest[citation needed] migration in American history occurred in the 1840s as the Latter-day Saints left the Midwest for the safety of the West. Both Omaha, Nebraska and St. Louis, Missouri laid claim to the title, "Gateway to the West" during this period. Omaha, home to the Union Pacific Railroad and the Mormon Trail, made its fortunes on outfitting settlers; St. Louis built itself upon the vast fur trade in the West before its settlement.

The 1850s were marked by political controversies which were part of the national issues leading to the Civil War, though California had been established as a non-slave state in the Compromise of 1850; California played little role in the war itself due to its geographic distance from major campaigns. In the aftermath of the Civil War, many former Confederate partisans migrated to California during the end of the Reconstruction period.
The California Gold Rush (1848–1855) began on January 24, 1848, when gold was discovered by James W. Marshall at Sutter's Mill, in Coloma, California.[1] News of the discovery soon spread, resulting in some 300,000 men, women, and children coming to California from the rest of the United States and abroad.[2] Of the 300,000, approximately 150,000 arrived by sea while the remaining 150,000 arrived by land.
The effects of the Gold Rush were substantial. San Francisco grew from a small settlement to a boomtown, and roads, churches, schools and other towns were built throughout California. A system of laws and a government were created, leading to the admission of California as a free state in 1850.
The Gold Rush started at Sutter's Mill, near Coloma.[3] On January 24, 1848 James W. Marshall, a foreman working for Sacramento pioneer John Sutter, found pieces of shiny metal in the tailrace of a lumber mill Marshall was building for Sutter, along the American River.[4]
Oregon Country or Oregon (to be distinguished from the American State also called Oregon) was a predominantly American term referring to a region of the Pacific Northwest of North America. The region was occupied by British and French Canadian fur traders from before 1810, and American settlers from the mid-1830s, with its coastal areas north from the Columbia River frequented by ships from all nations engaged in the fur trade, most of these from the 1790s through 1810s being Boston-based. The Oregon Treaty of 1846 ended disputed joint occupancy pursuant to the Treaty of 1818 and established the British-American boundary at the 49th parallel.