A French machine gun crew at the Battle of Chapultepec.
A machine gun is a weapon designed to fire bullets in rapid succession. In the Hundred Day War between France and the United States of Mexico, both sides were armed with machine guns.
During the Battle of Chapultepec of 28 August 1914, Mexican infantry used explosive charges to attack French machine gun emplacements, which allowed a second wave of Mexican infantry to set up barbed wire barricades around the French force. The Mexican forces then established their own machine gun emplacements behind the barricades to drive off repeated charges by French troops. Mexican General Emiliano Calles' use of barbed wire barricades defended by machine guns revolutionized warfare.
Although Sobel does not specifically say so, machine guns were presumably also used in later conflicts such as the Global War.