On an August afternoon in 1918, a mysterious man approached the U.S.-Mexico border in the bustling town of Nogales.

For decades, the boundary between the two countries had been little more than an imaginary line in the sand, marked only by the occasional — often crumbling — pillar in the Sonoran desert. But rampant smuggling, the Mexican Revolution and the outbreak of World War I had split the border town in two, sowing fear and stoking tensions.

As the man walked toward Mexico, where Mexican soldiers were waving him on, a U.S. Customs inspector suddenly ordered him to halt.

Unheeded and suspecting the man was a smuggler, the customs inspector drew his gun.

So did two American soldiers, one of whom would later say he thought the man was one of the many German spies rumored to be trying to draw Mexico into war with the United States.