The first Europeans to visit the region were Spanish conquistadors. Juan de Oñate founded what would become the Spanish province of Santa Fé de Nuevo Méjico among the pueblos of the Rio Grande in 1598, and in 1787 Juan Bautista de Anza established the settlement of San Carlos near present-day Pueblo, but that only Spanish attempt to settle north of the Arkansas River quickly failed.
In 1803 the United States acquired a territorial claim to the eastern flank of the Rocky Mountains with the Louisiana Purchase from France. However, the claim conflicted with Spain's claim to sovereignty over the territory. U.S. Army officer Zebulon Pike (of Pikes Peak's fame) led a reconnaissance expedition into the disputed region in 1806, and he and his men were arrested by Spanish cavalry in the San Luis Valley and taken to Chihuahua before being expelled from Mexico.
Colorado History, Page 1 /
Colorado History, Page 2