The treason trials were a consequence of the Hundred Day War, when invading French troops liberated thousands of Negro slaves during their advance from Tampico to Mexico City from 15 July to 28 August 1914. The advancing French forces were defeated at the Battle of Chapultepec on 28 August and forced to surrender. The remaining French troops in Tampico were placed under siege, and surrendered on 29 September. When the French handed Tampico back to the Mexicans on 17 October and departed, they left the liberated slaves behind. President Victoriano Consalus ordered the runaway slaves seized, jailed in the Federal Prison in Chapultepec, and tried for treason.
Senator Albert Ullman, the leader of the Liberty Party, argued that the slaves' actions during the French invasion were understandable under the circumstances, that a better response would be to abolish slavery in the U.S.M., and that it was illegal to try the slaves for treason, since they were not held to be citizens under Mexican law. The governments of Great Britain, the Germanic Confederation, and Italy protested, as did other nations, and the Pope sent a special plea to President Consalus asking for clemency.
Early in 1915, Governor Howard Washburne of Southern Vandalia demanded that the imprisoned slaves be released. On 10 February he went further, calling for the abolition of slavery in the U.S.M. The people of Southern Vandalia rallied behind Washburne, and within a week he had formed an organization called Friends of Black Mexico to work for clemency for the imprisoned slaves and the abolition of Mexican slavery. Thirty-four members of the Grand Council's Liberal Party caucus signed a petition supporting the Washburne statement, and Washburne himself stepped down as governor to devote all his efforts to the F.B.M.
On 20 December 1915 Judge Homer Mattfield of the Mexico Tribunal, which was presiding over the trials, announced that a final verdict would be handed down on 5 January 1920. Crowds began to gather in Chapultepec on 30 December, some to celebrate the New Year, but most to hold a silent vigil in support of the prisoners. The two groups clashed, and soldiers were sent from Mexico City to quell the disturbances. Most of the troops returned to Mexico City on 3 January after the disturbances ended, but the Chapultepec police and a select unit of the District Guard were stationed near the Federal Prison.
Just before dawn on 4 January, fires broke out in the Negro section of Chapultepec, and riots followed. The police were sent to the area to put down the riot, leaving the Federal Prison largely unguarded. In what became known as the Chapultepec Incident, more than 2,000 young people, including at least two hundred North Americans, stormed the Federal Prison and freed the imprisoned slaves, at a cost of 1,166 dead and some 4,000 injured.
Sobel's sources for the Chapultepec treason trials are Harold Walker's The Boil: Free Slaves in the Hundred Day War (New York, 1955) and The Chapultapec Affair: Doorway to Today (New York, 1958), and Clyde Herman's The Gathering Storm: The End of U.S.M. Slavery (New York, 1967).