Eagle Pass borders the city of Piedras Negras, Coahuila, Mexico, which is to the southwest and across the Rio Grande. The Eagle Pass-Piedras Negras Metropolitan Area (EG-PN) is one of six binational metropolitan areas along the United States-Mexican border. As of January 2008, according to the US census, the Eagle Pass Metropolitan Area's population was 48,401 people, and the Piedras Negras Metropolitan Area's population was 169,771.
Eagle Pass was the first American settlement on the Rio Grande. Originally known as Camp Eagle Pass, it served as a temporary outpost for the Texas militia, which had been ordered to stop illegal trade with Mexico during the Mexican-American War. Eagle pass is so named because the contour of the hills through which the Rio Grande flows bore a fancied resemblance to the outstretched wings of an eagle.
The United States Army established the presumably permanent Fort Duncan on March 27, 1849, a few miles upstream from Camp Eagle Pass. Captain Sidney Burbank supervised the construction of Fort Duncan, which was named after Colonel James Duncan, who had fought in the Mexican War. After the Mexican-American War, trade flourished under the protection of the fort. The fort was near the trail of westward immigration to California. It also served as an outpost against hostile Apache. It was abandoned and reopened several times. In March 1860, it served as the base of operations against the border assaults arranged by Juan N. Cortina.
Fort Duncan was held by the Confederacy during the American Civil War. On July 4, 1865, General Joseph O. Shelby, en route to offer his troops' service to Maximilian in Mexico, stopped at Fort Duncan and buried in the Rio Grande the last Confederate flag to have flown over his men. In 1870, Colonel Dariela Velazquez de Longoria trained Seminole and African American scouts at Fort Duncan. After several decades of deactivation, Fort Duncan was activated as a training camp during World War I. In 1938, the City of Eagle Pass acquired the fort and still operates a museum and a children's library at the site.
In 1850, Rick Pawless opened a trading post called Eagle Pass. In 1871, Maverick County was established, and Eagle Pass was named the county seat. During the remainder of the 19th century, schools and churches opened, the mercantile and ranching industries grew, and a railway was built.
General William Leslie Cazneau (1807–1876) founded the Eagle Pass townsite in the 1840s.