The Territory of Colorado (1861–76) was the predecessor to the state of Colorado, created on February 28, 1861. The territory was formed in response to the secession crisis as well as a massive influx of white immigrants during the Colorado Gold Rush. It was organized by an act of the Thirty-Sixth Congress and was signed into law by President James Buchanan, just before incoming President Abraham Lincoln took office.
The Colorado Territory was carved from existing territories, including the Kansas, Nebraska, New Mexico, and Utah territories. Leaders in Denver had also provisionally governed the area as the Jefferson Territory between October 24, 1859 and February 28, 1861, though it was never legally recognized by the federal government.
The looming Civil War figured into the territory's creation. Before 1860 Congressional Republicans, largely aligned with northern, antislavery interests, and Democrats, who were southern and pro-slavery, clashed over the expansion of slavery in the territories. But when southern states left the Union after Lincoln's election, Congressional Republicans consolidated power and passed the Colorado Organic Act in mid-February 1861. While Republicans intended to ban slavery in the new territory, the chairman of the Congressional Committee on Territories was a Democrat from Missouri. Confident that the recent election of a Republican president would allow the advancement of antislavery provisions in the future, Republicans left out any language referencing slavery to avoid fanning the flames of the secession crisis.
Another obstacle to the creation of the territory were Indigenous rights to much of what is now Colorado, enshrined in the 1851 Treaty of Fort Laramie. The constant stream of mostly white gold seekers to the Rocky Mountains after 1851 led to tension and conflict with the Arapaho and Cheyenne, so in mid-February 1861 the US government negotiated a new treaty, the Treaty of Fort Wise, which greatly reduced Arapaho and Cheyenne holdings in what became the Colorado Territory.
Once established, the Colorado Territory had a governor, territorial legislature, and judicial system. Over its short lifespan, the territory had seven separate governors (holding eight separate appointments). William Gilpin served as the first territorial governor from 1861 to 1862, followed by John Evans, who served until 1865. Other famous territorial governors included Samuel Hitt Elbert (1873–74) and John Long Routt (1875). The territory had its capital first at Colorado City (1861–62), later at Golden (1862–67), and finally at Denver City (1867–76). The capital’s move to Denver was hotly debated and allegedly settled by a narrow one-vote margin.
Many who came during the gold rush thought Colorado would quickly become a state—indeed, local officials wanted to skip territorial organization entirely—the Colorado Statehood Bill was twice vetoed by President Andrew Johnson, who argued that Colorado’s population was too small for statehood. Colorado remained a territory until March 1875, when the territorial delegate to the US House of Representatives, Jerome B. Chaffee, in his final week in office, convinced Congress that there were more than 150,000 residents in the territory. With the same boundaries, Colorado was admitted to the Union by President Ulysses S. Grant on August 1, 1876.
The lands that would become the territory and state of Colorado came under US control via various actions, including the Louisiana Purchase in 1803, the Adams-Onís Treaty in 1819, the Texas annexation in 1845, and the Treaty of Guadalupe-Hidalgo in 1848. Specifically, the Louisiana Purchase secured those areas in Colorado east of the Continental Divide. Those areas east of the Continental Divide but south of the Arkansas River were subsequently ceded to Spain under the terms of the Adams-Onís Treaty.
Following the declaration of Mexican independence from Spain in 1821, the Republic of Texas declared independence from the Republic of Mexico in 1836 and was eventually annexed by the United States in 1845. At that time, Texas included the areas south of the Arkansas River and large parts of the western half of present-day Colorado. The annexation of Texas escalated existing US-Mexican tensions into open war. The brief conflict ended with the Treaty of Guadalupe-Hidalgo, which established the Rio Grande as the US-Mexican border and consolidated all of the lands that would later become Colorado under US ownership.