In part to determine the southwestern boundary of the Louisiana Purchase, then held to be the Red River, Lieutenant Zebulon M. Pike was officially dispatched with a small party to explore the region. The expedition reached the present site of Pueblo on November 23, 1806, and Pike and a few men set out to climb the great peak that bears his name. For days it had been seen hanging like a white cloud on the horizon to the north, and the clarity of the atmosphere made it appear deceptively close. Lightly clothed and with little food, having to battle their way upward through a blizzard, Pike and his men failed to reach the summit, and in his journal Pike noted his opinion that the peak could not be scaled. He then moved westward up the Arkansas River to its source, retraced his steps some distance, and crossed the Sangre de Cristos into the San Luis Valley, proceeding westward to the Rio Grande, which he mistook for the Red River. Five miles up the Conejos, a tributary of the Rio Grande, the party encamped and built a fortified stockade. In February 1807, Pike learned that he was on foreign soil when Spanish soldiers appeared and placed him and his command under nominal arrest. Taken to Santa Fe, later to Mexico, he was subsequently released and died in battle during the War of 1812.

The southwestern boundary of the Louisiana Purchase was not determined until 1819 when it was agreed that it should run along the Arkansas River into the mountains and thence northward along the Continental Divide. At the same time President Monroe sent Major Stephen H. Long and a party to explore the region along the new boundary. Entering Colorado in 1820 by way of the South Platte River, the party moved south along the mountains to the Arkansas River, which a few of Long's men ascended as far as the Royal Gorge. Long conceived a poor opinion of the plains section of Colorado, dismissing it as "the Great American Desert" upon which nothing would ever grow. During this expedition the first recorded ascent of Pike's "unscalable" peak was led by Dr. Edwin James, scientist and historian of the party, whose reports served to acquaint the country with the western frontier, stirring the imagination of bolder and more restless spirits.

Plainsmen and Mountain Men were soon courageously pushing into this wilderness, penetrating every corner of it, following the streams high into the mountains in search of beaver and other prized fur-bearing animals. Trading posts sprang up at strategic points along the base of the foothills. The most celebrated of these first mercantile establishments and social centers of Colorado was that built near the present site of La Junta by the Bent and St. Vrain Company, a subsidiary of the American Fur Company. This adobe fortress, begun in 1828 and completed four years later, the largest in Colorado (see Tour 9A), was managed by William Bent, his three brothers, and their partner Ceran St. Vrain, reputedly a French nobleman. The Cheyenne, Arapaho, Kiowa, and Comanche came to trade at the post, and here was held the great council of 1840 at which these ancient enemies buried the hatchet and effected a permanent alliance. Here, too, at one time or another, came virtually every hero of actual or mythical prowess in the Old West—Jim Bridger, perhaps the greatest of the Mountain Men; Jim Baker and Tom ("Broken Hand") Fitzpatrick; "Uncle Dick" Wootton, with his bristling black hair and his shrewd eye for a good "swap" or other forms of profitable enterprise; and Jim Beckwourth, the mulatto who became a war chief of the Crow; the tribe became so enamored of him and so dependent upon his courage and skill, so Beckwourth said, that they sought to prevent his departure by poisoning him so that they might "keep him with them always."

Kit Carson, trapper, scout, and Indian agent, was a hunter at the post from 1831 to 1842, living much of his life in the vicinity, dying in 1868 at Fort Lyon, forty miles to the east. Traders and trappers brought their wives, either Mexican women from Santa Fe or Taos, or Indian squaws from almost every tribe west of the Mississippi, and their half-breed children romped and tumbled about the fort. Indian wives were frequently changed, and several or more squaws graced many a trapper's household. They were usually well-treated, however, and were "as happy as red paint and `froo-foo-raw' could make them." Thomas J. Farnham visited the fort in 1839 and has left a fascinating record of life there in his Western Travels, as has Francis Parkman in his Oregon Trail.

The post, a resting point for Kearney's Army of the West in 1846 and for Price's forces en route to Santa Fe in 1847, was abandoned in 1852 with the decline of the Indian trade; William Bent tried to sell the fort to the Federal Government as a military station, and when he failed, blew it up rather than have it occupied without payment. Down the Arkansas he built another post near the present site of Fort Lyon, where this greatest of Plainsmen continued in business until 1862, dying seven years later on a ranch near the Purgatoire River.

The Bents' most active rival was Louis Vasquez; in 1836 he and Andrew Sublette built a trading post for the Rocky Mountain Fur Company on the South Platte just north of the present site of Denver. Fort Vasquez was likewise a rendezvous of Plainsmen and Mountain Men ; although Vasquez was not as well liked as the Bents, he did a thriving business with the Northern Cheyenne and Arapaho, and with white trappers and hunters from the Ute country. In 1840 the post was sold to Locke and Randolph, and in 1842 it was captured and looted by Arapaho raiders. This post has recently been reconstructed. The trade in beaver pelts began to decline about 1840, and the posts, one by one, were abandoned. Their protective functions were assumed by military forts, the first of which, Fort Massachusetts, was built in 1852 for the protection of settlers in the San Luis Valley; it was soon abandoned for a better site at Fort Garland near by.

Between 1842 and 1853 Lieutenant John C. Fremont led five exploring expeditions into the Rocky Mountain region, the first three under the auspices of the Federal Government. On his expedition in 1842 Fremont followed the South Platte to Fort St. Vrain, struck northward to Fort Laramie, and returned east along the North Platte. The next year he crossed Colorado twice on his way to and from the Pacific. In 1848, while attempting to find a feasible railroad route through the Rockies, he and his men became snow-bound high in the La Garita Mountains of southern Colorado. After fearful privations Fremont and a few men succeeded in reaching Taos, New Mexico. The fifth expedition in 1853 took him through the San Luis Valley, over Cochetopa Pass, and into the basin of the Gunnison River, along much the same route followed earlier that year by Captain John W. Gunnison.

During this pre-settlement period the political status of Colorado changed frequently. As the Louisiana Territory was apportioned, the plains section north of the Arkansas was assigned in turn to Louisiana, Missouri, Nebraska, and Kansas. The remaining two-thirds of the State, south of the Arkansas and west of the Continental Divide, fell to Mexico when it achieved its independence from Spain in 1821. Later, when the Republic of Texas won its freedom from Mexico in 1836, the former laid claim to a strip of land extending north along the mountains to the 42nd parallel. The quarrel between the two southern republics over this territory continued after Texas had been admitted to the Union and did much to precipitate war between the United States and Mexico. By the Treaty of Guadalupe Hidalgo the United States acquired all of what is now Colorado, dividing its southern and western sections between New Mexico and Utah.

By the treaty the United States agreed to respect the property rights of all who had settled in the territory, for Mexico had made desperate attempts to preserve her northern possessions by colonizing them, awarding huge land grants to favored individuals, creating within the present State the princely demesnes of Maxwell, Nolan, Baca, and Sangre de Cristo. On this last grant the first permanent settlement in Colorado was made in 1853, near the present town of San Luis in the San Luis Valley. But settlers here or elsewhere in the region were few, for the traders and trappers who had tried ranching after the decline of the fur trade had not prospered, and only a handful remained on the plains or in the mountains by 1855. Colorado was still a wilderness, although destined to remain so but a short time.