Portions of what is now Wyoming were at one time claimed by Spain, France, and England. The acquisition of the territory by the United States was completed through five major annexations—the Louisiana Purchase in 1803, the Treaty of 1819 with Spain, cession by the Republic of Texas in 1836 and partition from Texas after it was annexed in 1845, the Treaty of Guadalupe Hidalgo (1848) after the Mexican War, and the international agreement (1846) with Great Britain concerning the Columbia River country (see Oregon).
The early development of Wyoming was closely linked with the fur trade and the great westward migrations. French trappers and explorers may have reached the area in the middle to late 18th cent., but the first authentic accounts of the region were provided by John Colter, who, trapped in the Wyoming mountains for several years, returned to St. Louis in 1810 with fantastic accounts of the steaming geysers and great canyons of the Yellowstone. Colter returned west, and other fur traders made their way into Wyoming. The overland party on its way to found Astoria on the Columbia River went through Teton Pass in 1811. The following year Robert Stuart, returning from Astoria, crossed South Pass and followed much of the route that was to become the Oregon Trail.
Only the hardiest and most self-sufficient could survive the Native American attacks and the rugged isolation of the country. With the expeditions of William H. Ashley, the mountain men entered the country, and some of the most famous of those early explorers—Thomas Fitzpatrick, James Bridger, and Jedediah S. Smith—crossed and recrossed the land. Attracted by the fur trade, Capt. B. L. E. de Bonneville organized a sizable expedition, and his were the first wagons to go (1832) through South Pass. The first permanent trading post was Fort William (1834), famous under its later name, Fort Laramie. In 1843 Fort Bridger (now in a state park) was built. The area also aroused the interest of John C. Frémont, who made an expedition in 1842. By the 1840s the route west through Wyoming was in steady use by caravans headed toward Oregon, and the fur-trading posts became stations on the Oregon Trail.
As the fur trade declined, many former trappers and mountaineers settled along the trail, furnishing horses and other supplies to the migrants and purchasing debilitated stock to be put to pasture and sold the following year. Mormons trekking to Utah (Brigham Young led the first party in 1847) and Forty-Niners rushing to the gold fields of California joined the many thousands traversing the mountain passes of Wyoming. A number of Mormons settled for a time in W Wyoming. The death of Mormon pioneers in a blizzard (1856) and the thousands of graves along the Oregon Trail give an indication of the toll taken by disease, starvation, attacks by Native Americans, and winter snows. Despite the hardships, telegraph stations (1861) and stagecoach and freight lines were established, and in 1860–61 pony express riders crossed Wyoming on their route between St. Joseph, Missouri, and Sacramento, California.
Native Americans hostile to encroachment in the early 1860s forced the rerouting of stagecoaches to the south, along the Overland Trail. Displaced from their former homes in the east and west, and waging internecine warfare for control of the rich buffalo ranges, the tribes feared further encroachment by the settlers on their hunting grounds, especially after the opening (1864) of the Bozeman Trail. Treaties were made and broken by both sides, and wars with the Sioux persisted, particularly in the Powder River valley.
Meanwhile, S Wyoming was relatively free of attacks, and a gold rush, stimulated by the discoveries at South Pass (1867), brought the first heavy influx of settlers to that region; the flow was increased by the uncovering of vast coal deposits in SW Wyoming. Probably the greatest stimulus to settlement was the completion (1868) of the Wyoming sector of the Union Pacific RR. Towns, including Cheyenne, sprang up beside the tracks, and trade thrived on the demands of the road crews and the new settlers.
In 1868 the region became the Territory of Wyoming, with Cheyenne as its capital. Wyoming pioneered in political equality when, in 1869, the first territorial legislature granted the vote to women. The territory continued to advance economically as huge herds of cattle were driven up over the Texas or Long Trail. Native American resistance had been suppressed by the late 1870s. The Arapaho were placed on the Wind River Reservation with their former enemies, the Shoshone, and cattlemen safely moved their herds to grasslands throughout Wyoming.
After the complete opening of rangelands, cattle rustling became so common that the authorities could not control it, and juries grew fearful of returning just verdicts against criminals. The Wyoming Stock Growers Association was organized in 1873 to protect cattle owners, and members frequently formed vigilante groups to administer their own justice. The struggle reached its height in the Johnson county cattle war of 1892. Lawlessness was also exemplified by the Hole-in-the-Wall gang, which broadened its activities to include bank and train robberies as well as cattle theft.
Gradually, vast areas were fenced in and winter pastures were established. The influx of sheep in the late 1890s, however, brought new violence. Cattlemen made frantic efforts to exclude the sheep from close grazing on the precious grasslands. Homesteaders were also unwelcome, and many left when they realized that the country was unsuited for small acreage cultivation. Nonetheless, population increase was steady, advancing from about 9,000 in 1870 to over 90,000 in 1900. With expanding population came other kinds of development: eager frontiersmen rapidly (and somewhat chaotically) established schools, and in 1887 the Univ. of Wyoming was founded.
Statehood was achieved in 1890, and in keeping with its frontier ideals, Wyoming adopted a liberal state constitution that included the secret ballot. The Carey Act of 1894, providing for the reclamation and settlement of land, stimulated further agrarian development and, in addition, pointed out the need for conservation and efficient use of water. The establishment of national parks protected timberlands and extensive grazing areas, and water power was harnessed to furnish electricity for farms and industries.
In politics, the Progressive movement found numerous adherents in Wyoming; in 1915, after one of the most bitter fights in the state's history, Progressive forces triumphed over the railroad and related interests with the establishment of a state utilities commission. A worker's compensation law was passed in 1915, and also in that year the legislature authorized the Univ. of Wyoming to accept federal grants for agricultural experiments and demonstrations. Thus were begun the state's outstanding and widespread services for agrarian improvement. In 1924 Wyoming became the first state to elect a woman governor, Nellie Tayloe Ross.
By the mid-1920s the state ranked fourth in the nation in the production of crude oil, but the valuable finds at Teapot Dome are probably remembered best as the symbol of corruption in the administration of President Warren Harding. Under the New Deal, Wyoming was well served by national soil conservation programs, which benefited dry farmers who had extended operations into semiarid regions and had suffered severely in the drought years beginning in the late 1920s. The cooperative movement in agriculture also gained ground in this period and has since grown.
One of the most important events in the state since World War II was the discovery of uranium. New oil finds also helped to offset economic losses resulting from a disastrous four-year-long drought in the 1950s. The decade from the early 1970s to the early 1980s was a boom period for Wyoming as high energy prices boosted the state's coal, oil, and natural gas industries. By the mid-1980s, however, energy prices were falling and the economy was hurt by its lack of diversity, but tourism and recreation subsequently developed as an important sector of the economy. Wyoming also has suffered from the injurious environmental effects of the energy industry, and pollution has become a serious problem in some mining towns. Although its population rose by almost 9% between 1990 and 2000, the state is still the least populous in the nation. With the increase in energy prices in the early 21st cent. Wyoming again found itself in an economic boom.