In 1843, Gilpin joined Kit Carson and several other notable westerners on John Charles Fremont's expedition to map the route over the Continental Divide as far as Oregon. A few years later, Major Gilpin marched his regiment to Chihuahua City during the Mexican-American War. He did well enough there that Gilpin received command of a volunteer force organized to suppress Indian uprisings in the West and to protect the Santa Fe Trail. After this, Gilpin settled in Independence, Missouri, where he set up a law practice and gave lectures on the health and wealth that was available in the Rocky Mountains.
At first, the merchants of Denver were all too happy to exchange their goods for Gilpin Scrip. However, Washington considered the scrip illegal and the U.S. Treasury refused to redeem them. Despite traveling to Washington to plead his case, the cabinet removed Gilpin from office by a unanimous vote. Ironically, Gilpin achieved his purpose. His illegally funded First Regiment distinguished itself, participating at the Battle of Valverde outside Fort Craig, and routing Confederate General Henry Sibley's Army at Glorieta Pass.
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One very entertaining book on Gilpin is Colorado: A History, by Marshall Sprague (1984).