The major portion of present-day Wyoming had become part of the United States as a result of the Louisiana Purchase from France in 1803. A small section in the west was included in the Oregon country, acquired by treaty with Great Britain in 1846. A small area in the south was acquired in 1845 as part of the Texas annexation, while the southwest corner came by cession from Mexico in 1848 after the Mexican War.

The main impetus to settlement came from the railroad. In 1867 the Union Pacific Railroad entered Wyoming, and settlement followed along its route. Discovery of gold at South Pass that year also brought settlers. Cheyenne, the first of the terminal towns, was founded in 1867; two years later it was the capital of the newly formed Territory of Wyoming. One of the first acts of the new territorial legislature was to give women the right to vote; it was the first governmental body in the world to grant women equal voting rights with men.

By 1877 all hostile bands of Indians had been driven from Wyoming, and the remaining Indians had been placed on reservations. During the 1870's, cattle ranching became the territory's most important industry. Sheep raising and general farming followed in importance. For years there was much strife between cattlemen and sheepmen and between cattlemen and farmers over grazing rights. This trouble culminated in the Johnson County, or Rustler, War of 1892, a vigilante action by prominent cattlemen and hired gunmen against farmers and others suspected of rustling. It ended after federal troops were called in to restore order.

On July 10, 1890, Wyoming became the 44th state of the Union. Cheyenne remained the capital. The new state government, like the territorial government, granted women the right to vote.