The Treaty of Guadalupe Hidalgo (which officially ended the Mexican-American War) and the Treaty of 1884 were the agreements originally responsible for the settlement of the international border, both of which specified that the middle of Rio Grande was the border – irrespective of any alterations in the channels or banks. The Treaty of 1884 went on to maintain that the alterations had to result from such gradual natural causes. This provision followed the long-established doctrine of international law that when changes in the course of a boundary river are caused by a deposit of alluvium, the boundary changes with the river, but when changes are due to avulsion, the old channel remains the boundary.
The river continually shifted south between 1852 and 1868, with the most radical shift in the river occurring after a flood in 1864. By 1873 the river had moved approximately 600 acres (2.4 km2), cutting-off land that was in effect made United States territory. The newly exposed land came to be known as El Chamizal, and eventually the land was settled and incorporated as part of El Paso. Both Mexico and the United States claimed the land. In 1895, Mexican citizens filed suit in the Juárez Primary Court of Claims to reclaim the land.
In 1899, both countries dug a channel across the heel of the horseshoe bend of the river at the dispute site for flood control purpose. This moved a 385-acre tract of land to the U.S. side of the river, but as man-made alterations do not change the boundary, this tract of land remained Mexican territory. This tract of land came to be known as Cordova Island, in a sense it was an island belonging to Mexico inside U.S. territory. Thus, there was little or no control by the local authorities, which created a haven for crime and opportunities for illegal crossings.
In 1909, Porfirio Díaz and William Howard Taft planned a summit in Ciudad Juárez and El Paso, a historic first meeting between a Mexican and a U.S. president and also the first time an American president would cross the border into Mexico. But tensions rose on both sides of the border over the Chamizal, the only link between El Paso and Ciudad Juárez, even though it would be considered neutral territory with no flags present during the summit. The Texas Rangers, 4,000 U.S. and Mexican troops, U.S. Secret Service agents, federal agents and U.S. marshals were all called in to provide security. Frederick Russell Burnham, the celebrated scout, was put in charge of a 250-person private security detail hired by John Hays Hammond, who in addition to owning large investments in Mexico was a close friend of Taft from Yale and a U.S. Vice-Presidential candidate in 1908. On October 16, the day of the summit, Burnham and Private C.R. Moore, a Texas Ranger, discovered a man holding a concealed palm pistol standing at the El Paso Chamber of Commerce building along the procession route. Burnham and Moore captured, disarmed, and arrested the assassin within only a few feet of Díaz and Taft.