Before the late 1910s, International Street was exactly what its name suggested: a wide-open boulevard connecting two cities that happened to occupy different countries. Nogalenses moved freely between Arizona and Sonora, shopping, working, and visiting family without documentation. The Mexican Revolution changed everything. Violence from the fighting spilled across the border during battles in 1913 and 1915. Then came World War I and the Zimmermann Telegram, revealing Germany's attempt to draw Mexico into the conflict. American paranoia intensified. By August 1918, passport-carrying Mexican laborers could cross only twice daily. Workers dependent on American wages faced impossible choices. A Nogales newspaper noted that local merchants saw panicky times ahead as their customer base vanished behind new restrictions.

Two deaths in the months preceding August poisoned relations between the border guards. On New Year's Eve 1917, Francisco Mercado, an off-duty Mexican customs agent, tried to cross into Arizona. An American sentry called for him to stop, in English. Mercado kept walking. The soldier shot him dead before multiple witnesses. Then came Gerardo Pesqueira, son of a former Sonoran governor. He was deaf and mute. When sentries ordered him to halt, he could not hear them. They opened fire anyway. Pesqueira was known for his caring and cheerful nature, according to later accounts. The killers faced no consequences. Mexican border agents seethed at the impunity. American investigators would later conclude that the routine mistreatment of Mexican border crossers created the resentment that exploded on August 27th.

The shot that started everything may have been a warning fired into the air. Private William Klint of the 35th Infantry raised his Springfield rifle to force Gil Lamadrid back for inspection. Mexican customs officer Francisco Gallegos saw his countryman drop to the ground and grabbed his pistol, killing Klint with a shot to the face. American inspector Arthur Barber returned fire, dropping Gallegos and a fellow officer. Citizens on the Mexican side heard gunfire and ran for their rifles. Buffalo Soldiers of the 10th Cavalry charged from Camp Stephen Little, crossing into Sonora under heavy fire to secure hilltops overlooking the town. Captain Joseph Hungerford died leading the cavalry charge, a bullet through his heart. Women from the red-light district emerged with bedsheets marked as improvised red crosses, dragging wounded men to safety despite their own injuries.

By 7:45 that evening, with their mayor dead and U.S. troops holding the high ground, Mexican officials raised a white flag over their customs building. Lieutenant Colonel Frederick Herman, himself wounded in the thigh during the fighting, ordered an immediate ceasefire. Snipers on both sides kept shooting for a while longer before their leaders silenced them. The American toll: four dead, twenty-nine wounded, one of whom would die later. The Mexican casualties were higher, though counts varied. Sonoran Governor Plutarco Elias Calles, who would later become Mexico's president, arrived to negotiate. Both sides expressed regret. An American customs inspector was found guilty of improper conduct and removed for his harsh treatment of Mexicans.

General DeRosey Cabell's investigation recommended a two-mile fence down the middle of International Street. He wrote that it would do more to prevent friction than any other measure. Governor Calles agreed. The Nogales Herald reported that both officials believed the August clash and two previous incidents that year would have been averted had the fence existed. That chain-link barrier marked the end of free movement between the twin cities. In 1961, Mexico granted Nogales, Sonora the title of Heroic City for its citizens' actions that August day. A monument stands near the border commemorating the Mexican dead. On the American side, no memorial marks where the battle that built the first border wall began.