The outbreak of the Mexican Revolution in 1910 against the long-time rule of President Porfirio Díaz initiated a decade-long period of high-intensity military conflict along the U.S.–Mexico border, as different political/military factions in Mexico fought for power.

The violent aftermath of Madero's assassination during a coup in 1913 again highlighted the importance of the U.S.-Mexico border, as battles for control of Mexican Nogales between Villistas and Carrancistas (forces of Gen. Venustiano Carranza, a former Villa ally) led to American involvement because of cross-border firing into the U.S..

During the November 1915 Battle of Nogales fought between the forces of Francisco Villa and Carranza (led by Gen. Álvaro Obregón and Gen. Plutarco Elías Calles), one U.S. serviceman, Pvt.

This led directly to further border tensions as U.S. President Woodrow Wilson unilaterally dispatched the Punitive Expedition, under Gen. John Pershing, into the state of Chihuahua to apprehend or kill Villa.

After the U.S. entered World War I, the 10th Cavalry was based at Fort Huachuca, Arizona, with elements of that regiment also being stationed in Camp Stephen Little, the army post just north of Nogales.

Though recognized as the legitimate leader of the Mexican Republic, President Carranza did not control large swaths of territory—such as the regions held by Francisco Villa and Emiliano Zapata.

Moreover, the U.S. use of motor vehicles and two airplanes during the Punitive Expedition stood in stark contrast to the conditions existing within the Mexican Federal Army and the various disparate militias, where weapons, bullets, uniforms and even food could often be in very short supply.

[citation needed] By August 1918 the U.S. State Department had tightened wartime control at the border by limiting passport-carrying Mexican laborers to two entries per day and restricting non-workers to one crossing per week.

On the afternoon of 31 December 1917 Francisco Mercado, an off-duty Mexican customs agent, attempted to cross into Nogales, Arizona, despite calls from a U.S. Army sentry who asked him, in English, to stop.

The seeming impunity of U.S. border guards was becoming increasingly intolerable to nogalenses, a point made by Gen. Cabell and U.S. Consul in Nogales, Sonora, E.M. Lawton and Vice-consul W.A.

[10] However, in a brief passage from his 1921 book History of the Tenth Cavalry, 1866-1921 Edward Glass indicates that changes in Mexican officials and soldier attitudes helped contribute to the tense situation.

It reportedly stated the person was sickened and disgusted at the atrocities committed by Villa and his men, and without pay or reward, because of "friendly respect" for American troops, warned them of the German financial efforts and influences at work near and in Nogales.

The investigation of the Battle of Ambos Nogales instead traced the origins of the violence to the abusive practices of U.S. customs officials and the resentment caused by the killings along the border during the previous year.

In the written transcripts of the investigators' interviews with Lt. Col. Herman, the local commander made no mention whatsoever of the letter he later claimed to have received from the "unknown" and disgruntled Villista defector.

[14] On 27 August 1918, at about 4:10 pm, a gun battle erupted unintentionally when a civilian Mexican carpenter named Zeferino Gil Lamadrid attempted to pass through the border back to Mexico, without having the bulky parcel he was carrying with him inspected at the United States Customs house.

Although it was later speculated that most of the combatants were soldiers of the Mexican Federal Army, in fact part of the Nogales garrison was away fighting rebels opposed to Gen. Plutarco Elías Calles' governorship of Sonora.

[1][6] In the frenzy of the unexpected battle, armed Mexican civilians stormed the home of Gen. Álvaro Obregón on International Street and used its sturdy stone walls as a strongpoint from which to fire at U.S. targets.

"[21] As the troops advanced into the city, many of these women ventured out with bedsheets marked with impromptu red crosses in an effort to rescue persons wounded in the fighting.

[1][6] Allyn Watkins, an eyewitness to the shooting from the rooftops of homes along a tall hill on the U.S. side, claimed that the disordered involvement of U.S. civilians in the border fight "didn’t help the progress of the 'war' any".

Despite later accounts to the contrary by U.S. military personnel (including Lt. Col. Herman), an official note from the U.S. Consulate in Nogales, Sonora, confirmed that a shot "from the Arizona side" felled the Mexican mayor.

"I met with General Calles at 3 o’clock, having previously received a telegram from him expressing regret at the unfortunate incident of yesterday afternoon", Cabell remarked.

Cabell reiterated the U.S. demand that Sonoran officials halt the sporadic shooting from the Mexican side of the border, to which Calles said that the shooters were "irresponsible men" and beyond his control.

[29] Cabell conducted an investigation in which he and his associates interviewed a range of civilians (including U.S. Consul Lawton) and military personnel in an attempt to determine what caused the border violence that 27 August.

After completing his investigation, Gen. Cabell informed his commanding officer that an unnamed U.S. customs inspector had been found guilty of "improper conduct" and removed from duty because of his harsh treatment of Mexicans.

The investigation laid the blame, albeit begrudgingly, for the outbreak of violence in Ambos Nogales on resentment among nogalenses over the routine mistreatment of Mexican border crossers.

Oliver Fannin, Lieutenant, G Company, 35th Infantry, was awarded the Distinguished Service Cross "For valor and bravery ... while under fire, carried a wounded man to safety in the Nogales battle."

However, the official report by the Mexican government lists the dead at 15, with special attention given to the actions of Francisco Gallegos, Andres Ceceña and the fallen mayor Felix B. Peñaloza (the latter of whom has been consistently ignored by the few U.S. observers who have written about the battle).

[34] The U.S. government's investigation into the Battle of Ambos Nogales indicated that the origins of the violence were found in the resentment Mexican nationals felt from the U.S. Customs officials' poor treatment and the sense of impunity that took place when the killers of Francisco Mercado and Gerardo Pesqueira went unpunished.

Echoing the comments of some U.S. participants in the battle, James P. Finley wrote in Huachuca Illustrated "found among the Mexican dead were the bodies of two German agents provocateurs.

[39] In addition to the physical reminders of the Battle of Ambos Nogales through monuments and folklore, the presence of the current US–Mexico border fence running through the community owes its existence at least in part to the events of 27 August 1918.