| 1785 | Land Ordinance for Northwest Territory passed by Congress |
| 1786 | Jay-Gardoqui negotiations over Mississippi navigation anger southern states |
| 1803 | Louisiana Purchase concluded with France (May) |
| 1803-1806 | Lewis and Clark explore the Northwest |
| 1819 | Adams-Onis treaty cedes Spanish territory to the United States |
| 1820 | Missouri Compromise resolves nation’s first sectional crisis |
| 1823 | Earliest American settlers arrive in Texas |
| 1836 | Texas becomes independent republic |
| 1842 | Webster-Ashburton Treaty fixes border between Maine and New Brunswick |
| 1843 | Mass migration to Oregon begins |
| 1845 | United States annexes Texas, John L. O'Sullivan coins slogan Manifest Destiny |
| 1846 | United States and Great Britain resolve diplomatic crisis over Oregon |
| 1847 | American conquest of California completed |
| 1847 | Mormons settle Utah |
| 1848 | Treaty of Guadalupe Hidalgo consigns California and New Mexico to United States |
| 1848 | Gold discovered in California |
| 1849 | “Forty-niners" rush to California to dig for gold |
| 1859 | Gold and silver discoveries, in Colorado and Nevada |
| 1862 | Congress passes Homestead Act, encouraging western settlement |
| 1867 | Horace Greeley urges Easterners to "Go West, young man” |
| 1873 | Congress passes Timber Culture Act, Big Bonanza discovered on the Comstock Lode in Nevada |
| 1874 | Joseph F. Glidden invents barbed wire, Discovery of gold in Dakota Territory sets off Black Hills Gold Rush |
| 1883 | Museum expedition discovers fewer than two hundred buffalo in the West |
| 1886-1887 | Severe drought and winter damage cattle and farming bonanzas |
| 1897 | Gold discovered in Alaska |
| 1902 | Congress passes National Reclamation Act (the Newlands Act) |
| 1910 | Taft fires Gifford Pinchot, head of U.S. Forest Service |
| 1989 | Exxon Valdez oil spill pollutes over 500 square miles of Alaskan waters (March), San Francisco rocked by massive earthquake (October) |