Fort Duncan, on the east side of the Rio Grande above Eagle Pass in Maverick County, was established by order of Maj. Gen. William J. Worth on March 27, 1849, when Capt. Sidney Burbank occupied the site with companies A, B, and F of the First United States Infantry. John Twohig owned the 5,000-acre site. At the start of the Mexican War in 1846 a temporary post called Camp Eagle Pass had been established at the site. Roads ran from there to Fort Inge and Fort McIntosh, and mail was received from San Antonio. On November 14, 1849, the post was named Fort Duncan, after James Duncan, a hero of the Mexican War. The post consisted of a storehouse, two magazines, four officers' quarters, and a stone hospital, in addition to quarters for enlisted men. Construction was done half by the troops and half by hired workers. There was ample stone but no timber for building, and the men suffered from exposure. Company C, First Artillery, asked permission to construct quarters at its own expense. The fort was significant because of the trade crossing into Mexico at Eagle Pass, its location on the California Road, and its position for scouting against Indians in the 1850s.
Fort Duncan became involved in the Callahan expedition of 1855, when James H. Callahan led an effort to repel attacks of Lipan Apaches and to capture runaway slaves. Callahan seized and burned the Mexican town of Piedras Negras, and the commanders at the fort ultimately refused to help him recross the Rio Grande into the United States. Secretary of War John B. Floyd ordered the post abandoned in May 1859, and troops were transferred to Camp Verde on June 18. Because of Juan N. Cortina's disturbances on the Rio Grande, Robert E. Lee ordered the fort regarrisoned in March 1860, and in August 1860 Maj. D. H. Vinton leased the site from Twohig. With the outbreak of the Civil War the post was again abandoned when the federal troops evacuated on March 20, 1861, and John C. Crawford received the property as agent for the state of Texas. During the war the post was known as Rio Grande Station and served the Frontier Regiment. It was also an important customs point for Confederate cotton and munitions trade with Mexico. On June 19, 1864, the fort was attacked by a force of about 100 Mexicans from the surrounding area and Piedras Negras, but they managed to capture only a few horses.
Federal troops reoccupied Fort Duncan on March 23, 1868. In 1870 Seminole Indians were attached to the command as guides. The post was abandoned in 1883, when the government could not secure its purchase. Fort Duncan was renamed the Camp at Eagle Pass that year and served as a subpost of Fort Clark, housing Company A of the Third Cavalry. In 1894, however, the government finally bought the site. From 1890 to 1916, when disturbances in Mexico took national guard units to the river, the fort had a skeleton caretaking detachment. However, in 1916 the number of troops at the fort was increased to 16,000. Troop activity continued throughout World War I, when the fort served as a training facility, but by 1920 only a small detail remained. In 1933 the city of Eagle Pass began maintaining the old fort as a public park on the condition that the federal government could reclaim the post for military or other reasons. The city formally acquired the property in 1935 and converted it into Fort Duncan Park. In 1942 the mayor offered the fort to the military for use during World War II. The government accepted and used the Fort Duncan Country Club as an officers' club and the swimming pool for commissioned personnel stationed at Eagle Pass Army Air Field. The site was listed in the National Register of Historic Places in 1971. Seven of the original buildings were extant and had been restored. The old headquarters building became a museum.