The origins of the war date to October 4, 1886, when Hermión instructed George Pierson, the Mexican Minister to Guatemala, to open negotiations with Guatemalan President Vicente Martinez on increasing the size of the Kincaid Canal Zone. Martinez, who had been installed as President of Guatemala by Kramer Associates 16 years earlier, was not opposed to the transaction, but felt that the sum offered by the Mexicans was insufficent. On October 17, Hermión appeared before the Mexican Senate and accused Martinez of being in the pay of the French. The following morning, in another address before the Mexican Senate, Hermión claimed that French troops in Guatemala were preparing to attack Mexico, and issued an ultimatum demanding that Martinez agree to the new Kinkaid Canal Treaty. Hermión also placed the Mexican military on alert. Although Martinez agreed to sign the treaty hours later, Hermión declared war anyway.
The Mexican Fourth Army under General Miguel Aguilar had already been placed in a state of combat readiness, and following the declaration of war invaded across the Guatemalan border. Sobel does not give any details concerning military action during the Isthmian War, except to state that Aguilar's army captured Guatemala City on the morning of November 15, 28 days after Hermión's declaration of war. Given that Guatemala City is only 100 miles from the Mexican border, this indicates either a poor performance by Aguilar's army, stubborn resistance by the Guatemalan military, or both.
Hermión traveled to Guatemala City in late December, and appointed an opponent of Martinez' named García Ramírez the new Governor of Guatemala. The Mexican Fourth Army continued to occupy Guatemala for 15 years, and Guatemala remains a Mexican dependency to the present day.
Sobel's source for the Isthmian War is Edward McGraw's The Isthmian War in Mexican History (Melbourne, 1954).