The region that is today the State of Colorado has been inhabited by Native Americans for more than 13 millennia. The Lindenmeier Site in Larimer County contains artifacts dating from approximately 11200 BCE to 3000 BCE. The eastern edge of the Rocky Mountains was a major migration route that was important to the spread of early peoples throughout the Americas. The Ancient Pueblo Peoples lived in the valleys and mesas of the Colorado Plateau. The Ute Nation inhabited the mountain valleys of the Southern Rocky Mountains and the Western Rocky Mountains. The Arapaho Nation and the Cheyenne Nation moved west to hunt across the High Plains.
The United States acquired a territorial claim to the eastern flank of the Rocky Mountains with the Louisiana Purchase from France in 1803. This U.S. claim conflicted with the claim of Spain to a huge region surrounding its colony of Santa Fé de Nuevo Méjico as its sovereign trading zone with native peoples. Zebulon Pike led a U.S. Army reconnaissance expedition into the disputed region in 1806. Colonel Pike and his men were arrested by Spanish cavalrymen in the San Luis Valley the following February, taken to Chihuahua, and then expelled from Mexico the following July.
The United States relinquished its claim to all land south and west of the Arkansas River and south of 42nd parallel north and west of the 100th meridian west as part of its purchase of Florida from Spain with the Adams-Onís Treaty of 1819. The treaty took effect February 22, 1821. Having settled its border with Spain, the United States admitted the southeastern portion of the Territory of Missouri to the Union as the State of Missouri on August 10, 1821. The remainder of the Missouri Territory, including what would become northeastern Colorado, became unorganized territory, and would remain so for 33 years over the question of slavery. After 11 years of war, Spain finally recognized the independence of Mexico with the Treaty of Córdoba signed on August 24, 1821. Mexico eventually ratified the Adams-Onís Treaty in 1831. The Texian Revolt of 1835–1836 fomented a dispute between the United States and Mexico which eventually erupted into the Mexican-American War in 1846. Mexico surrendered its northern territory to the United States with the Treaty of Guadalupe Hidalgo at the conclusion of the war in 1848.
Most American settlers traveling overland west to the Oregon Country, the new goldfields of California, or the new Mormon settlements of Deseret in the Salt Lake Valley, avoided the rugged Southern Rocky Mountains, and instead followed the North Platte River and Sweetwater River to South Pass, the lowest crossing of the Continental Divide between the Southern Rocky Mountains and the Central Rocky Mountains. In 1849, the Mormons of the Salt Lake Valley organized the extralegal State of Deseret, claiming the entire Great Basin and all lands drained by the Green, Grand, and Colorado rivers. The federal government of the United States flatly refused to recognize the new Mormon government, because it was theocratic and sanctioned plural marriage. Instead, the Compromise of 1850 divided the Mexican Cession and the northwestern claims of Texas into a new state and two new territories, the State of California, the Territory of New Mexico, and the Territory of Utah. On April 9, 1851, Mexican American settlers from the area of Taos settled the village of San Luis, then in the New Mexico Territory, later to become Colorado's first permanent Anglo American settlement.
In 1854, Senator Stephen A. Douglas persuaded the U.S. Congress to divide the unorganized territory east of the Continental Divide into two new organized territories, the Territory of Kansas and the Territory of Nebraska, and an unorganized southern region known as the Indian territory. Each new territory was to decide the fate of slavery within its boundaries, but this compromise merely served to fuel animosity between free soil and pro-slavery factions.
Gold was discovered along the South Platte River then in western Kansas Territory in July 1858, precipitating the Pike's Peak Gold Rush. The placer gold deposits along the rivers and streams of the region rapidly played out, but miners soon discovered far more valuable seams of hard rock gold in the nearby mountains.
The gold seekers organized the Provisional Government of the Territory of Jefferson on August 24, 1859, but this new territory failed to secure approval from the Congress of the United States embroiled in the debate over slavery. The election of Abraham Lincoln for the President of the United States on November 6, 1860, led to the secession of nine southern slave states and the threat of civil war among the states. Seeking to augment the political power of the Union states, the Republican Party dominated Congress quickly admitted the eastern portion of the Territory of Kansas into the Union as the free State of Kansas on January 29, 1861, leaving the western portion of the Kansas Territory, and its gold-mining areas, as unorganized territory.
Thirty days later on February 28, 1861, outgoing U.S. President James Buchanan signed an Act of Congress organizing the free Territory of Colorado. The original boundaries of Colorado remain unchanged today. The name Colorado was chosen because it was commonly believed that the Colorado River originated in the territory. Early Spanish explorers named the Colorado River the "Rio Colorado" for the red-brown silt that the river carried from the mountains. In 1859, a U.S. Army topographic expedition led by Captain John Macomb located the confluence of the Green River with the Grand River in what is now Canyonlands National Park in Utah. The Macomb party designated the confluence as the source of the Colorado River. In fact, the river that is now named the Colorado River did not flow through Colorado until the House of Representatives Joint Resolution 460 of the 66th United States Congress changed the name of the Grand River to the Colorado River on July 25, 1921, in spite of the fact that the Green River had the more extensive drainage basin.
On April 12, 1861, South Carolina artillery opened fire on Fort Sumter to start the American Civil War. While many gold seekers held sympathies for the Confederacy, the vast majority remained fiercely loyal to the Union cause. In 1862, a force of Texas cavalry invaded the Territory of New Mexico and captured Santa Fe on March 10. The object of this Western Campaign was to seize or disrupt the gold fields of Colorado and California and to seize ports on the Pacific Ocean for the Confederacy. A hastily organized force of Colorado volunteers force-marched from Denver City, Colorado Territory, to Glorieta Pass, New Mexico Territory, in an attempt to block the Texans. On March 28, the Coloradans and local New Mexico volunteers stopped the Texans at the Battle of Glorieta Pass, destroyed their cannon and supply wagons, and ran off 500 head of their horses and mules. The Texans were forced to retreat to Santa Fe. Having lost the supplies for their campaign and finding little support in New Mexico, the Texans abandoned Santa Fe and returned to San Antonio in defeat. The Confederacy made no further attempts to seize the Southwestern United States.
In 1864, Territorial Governor John Evans appointed the Reverend John Chivington as Colonel of the Colorado Volunteers with orders to protect white settlers from Cheyenne and Arapaho warriors who were accused of stealing cattle. Colonel Chivington ordered his men to attack a band of Cheyenne and Arapaho encamped along Sand Creek. Chivington reported that his troops killed more than 500 warriors. The militia returned to Denver City in triumph, but several officers reported that the so called battle was nothing more than a massacre of Indians at peace, that most of the dead were women and children, and that bodies of the dead had been mutilated and desecrated in hideous manner. Three U.S. Army inquiries condemned the action, and incoming President Andrew Johnson asked Governor Evans for his resignation, but none of the perpetrators was ever punished.
In the midst and aftermath of Civil War, many discouraged prospectors returned to their homes, but a determined few stayed on to develop mines, mills, farms, ranches, roads, and towns in the Territory. On September 14, 1864, James Huff discovered silver near Argentine Pass, the first of many silver strikes. In 1867, the Union Pacific Railroad laid its tracks west to Weir, now Julesburg, in the northeast corner of the Territory. The Union Pacific linked up with the Central Pacific Railroad at Promontory Summit, Utah, on May 10, 1869, to form the First Transcontinental Railroad. The Denver Pacific Railway reached Denver in June of the following year, and the Kansas Pacific arrived two months later to forge the second line across the continent. In 1872, rich veins of silver were discovered in the San Juan Mountains on the Ute Indian reservation in southwestern Colorado. The Ute people were removed from the San Juans the following year.
The United States Congress passed an enabling act on March 3, 1875, specifying the requirements for the Territory of Colorado to become a state. On August 2, 1876 (28 days after the Centennial of the United States), U.S. President Ulysses S. Grant signed a proclamation admitting Colorado to the Union as the 38th State and earning it the moniker "Centennial State".
The discovery of a major silver lode near Leadville in 1878, triggered the Colorado Silver Boom. The Sherman Silver Purchase Act of 1890 invigorated silver mining, and Colorado's last, but greatest, gold strike at Cripple Creek a few months later lured a new generation of gold seekers. Colorado women were granted the right to vote beginning on November 7, 1893, making Colorado the second state to grant universal suffrage and the first one by a popular vote (of Colorado men). The repeal of the Sherman Silver Purchase Act in 1893 led to a staggering collapse of the mining and agricultural economy of Colorado, but the state slowly and steadily recovered.
Colorado became the first western state to host a major political convention when the Democratic Party met in Denver in 1908. By the U.S. Census in 1930, the population of Colorado first exceeded one million residents. Colorado suffered greatly through the Great Depression and the Dust Bowl of the 1930s, but a major wave of immigration following World War II boosted Colorado's fortune. Tourism became a mainstay of the state economy, and high technology became an important economic engine. The United States Census Bureau estimated that the population of Colorado exceeded five million in 2009.
Three warships of the U.S. Navy have been named the USS Colorado. The first USS Colorado was named for the Colorado River. The later two ships were named in honor of the state, including the battleship USS Colorado which served in World War II in the Pacific beginning in 1941. At the time of the Attack on Pearl Harbor, this USS Colorado was located at the naval base in San Diego, Calif. and hence went unscathed.