By the close of the 19th century, railroads and telegraph lines spanned the American continent. Settlement had spread to such an extent throughout the West that the Census of 1890 announced there was no longer a frontier line. Without the “frontier beyond,” Americans began to view the wilderness differently. It was not, after all, endless and inexhaustible, but finite, shrinking and worthy of protection. The most effective spokesman for the cause of western wilderness preservation in the latter decades of the 19th century was the Scottish-born naturalist, John Muir (1838-1914). After studying at the University of Wisconsin, Muir hiked all over the West, studying and writing about the region’s wild places, especially its mountains, forests and glaciers. In arguing for the preservation of nature in its primeval state, Muir was echoing views expressed earlier by Thoreau and Emerson, but he articulated them with new intensity. Muir was particularly entranced by the beauty of California’s Yosemite Valley and its surrounding Sierra Neva mountains, which he called the “range of light.” At his urging, the area was finally designated a national park in 1890, the nation’s third after Sequoia (1890) and Yellowstone (1872). In 1892, Muir founded the Sierra Club, an organization dedicated to the preservation and enjoyment of the wilderness.

The Mexican War of 1846-1848 broke out ostensibly over a dispute about the Texan boundary. But it really originated in the expansionist spirit of “manifest destiny,” of which President James K. Polk was a leading proponent. Polk came to office determined to acquire the Mexican provinces of California and New Mexico and when Mexico refused to sell them, used a border clash as a pretext for war. Although American opinion was deeply divided over the conflict, the war turned out to be a succession of triumphs for American arms. The two provinces Polk coveted were easily overrun: General Zachary Taylor overwhelmed a much larger Mexican army at Buena Vista (February 1847); and General Winfield Scott’s seaborne expedition to Vera Cruz fought its way into the heart of Mexico against superior forces and captured Mexico City (September 1847), forcing Mexico to make peace. By the Treaty of Guadalupe-Hidalgo (February 1848), Mexico ceded the territories of California and New Mexico and acknowledged the Rio Grande as the Texas boundary. While the war brought the U.S. vast territorial gains — extending America’s western boundary to the Pacific — it also revived the simmering conflict over slavery and its extension to new territories.