Lewis and Clark discovered the Salmon and Clearwater country of Idaho in 1805 -1806, and fur trappers explored the remainder of Idaho in search of beaver. The area's Snake River became a disputed border between British and French trappers from Canada and the lower Columbia River valley, vying with Americans from St. Louis.

Permanent white settlements began in 1860, when Mormons moved northward from Utah and miners arrived from the Pacific Coast. A series of Idaho gold rushes lured many people to the area about 1860. Idaho then was part of the Washington Territory. After territories were formed in Montana in 1864 and Wyoming in 1868, Idaho's boundaries enclosed two areas divided by mountains.

For more than 25 years, North Idaho attempted unsuccessfully to form a new territory with eastern Washington. In 1886-1887, the U.S. Congress voted to annex North Idaho to Washington Territory, but President Grover Cleveland was pressured not to approve that change. Idaho Territory was overwhelmingly Confederate Democratic after 1864. Then a coalition of Republicans and anti-Mormons took power in 1882. By 1888 Idaho had become "safely" Republican. Trying to gain control over the U.S. Congress, the Republicans voted for admission of the territory. Idaho was made a state on July 3, 1890.