Some of the dominant historical themes in the first half of the nineteenth century were territorial exploration and expansion, growing tension around the issue of slavery (exacerbated by the annexation of new territory), and industrialization and technological development, including progress in transportation and communication. The New England Transcendentalists were keenly aware of these historical currents.
The country grew rapidly, sanctioned by the 1823 Monroe Doctrine, in which President James Monroe had declared North America no longer open to European acquisition and meddling, and by the concept of "manifest destiny," articulated in the 1840s, proclaiming America's mission to spread its culture and government across North America. The Louisiana Purchase of 1803 added most of the area between the Mississippi and the Rockies. Between 1804 and 1806, Lewis and Clark explored the Northwest from the Mississippi to the Pacific. In 1806, Zebulon Pike traveled through the Southwest, to New Mexico and Mexico City. During the 1820s, traders hauled goods over the Santa Fe Trail. The first American settlement in Texas, which was then under Mexican control, was established in 1821. Americans subsequently flocked to Texas until 1830, when Mexico passed more restrictive immigration laws. California and Oregon were explored in the 1820s. Jedediah Strong Smith led an expedition to Mexican-held California in 1826. Oregon Country, jointly controlled by the United States and Great Britain, was explored in the late 1820s, settled in the 1840s and 1850s. Gold drew thousands westward, to California (beginning in 1848), to Colorado and Nevada (beginning in 1859), to Idaho (beginning in 1860) and Montana (beginning in 1862).
John C. Frémont undertook a series of expeditions in the 1840s, to track the headwaters of the Des Moines River (1841), to explore the route to Oregon (1842 and 1843–1844), and to California, to survey the Rockies and the area around the Great Salt Lake (1845–1846). Under the leadership of Brigham Young, the beleaguered Mormons began their migration from Illinois in 1846, arriving at the Great Salt Lake in 1847.
