Horn's path to Vanceboro was almost absurdly circuitous. When war broke out in 1914, he was managing a coffee plantation in Moka, Guatemala, having spent ten years in the German Army before taking a furlough in 1909 to work in Central America. Eager to return to Germany and serve, he traveled to British Honduras, sailed to Galveston, Texas, and reached New York -- only to find the British blockade of the North Sea made passage to Europe impossible. After a month of failed attempts, he went to Mexico City, discovered someone had taken his plantation job, found work at another plantation in Salto de Agua, Chiapas, and then received a message ordering him back to Germany. He returned to New York in late December 1914, checked into the Arietta Hotel, and there met Franz von Papen, the German military attache in Washington. Von Papen was recruiting saboteurs to disrupt Canadian railways and recognized in the zealous, displaced Horn an ideal tool.

Von Papen explained that bombing the bridge would be seen as an act of courage in Germany and assured Horn that no one would be killed. Both claims were misleading. The bridge carried heavy traffic, and there was a real chance a train would be caught in the blast. Von Papen wrote Horn a check for $700 on January 18, 1915 -- a paper trail that would eventually help unravel the entire German sabotage network in America. On January 29, Horn left Grand Central Terminal on a New Haven Railroad train to Boston, his suitcase of dynamite riding in the passenger car beside him. In Boston, he boarded the overnight train operated by the Boston and Maine Railroad, placing the explosives in a lower sleeping berth. The car was transferred to the Maine Central Railroad at Portland and continued east across the state to Vanceboro, the railroad's terminal at the Canadian border. Horn arrived, checked in, and spent two days watching the busy CPR main line to learn the train schedule.

On the night of Monday, February 1, 1915, Horn checked out of the Exchange Hotel, claiming to be catching a train. Instead, he changed into his German army uniform -- a calculated move to avoid being classified as a spy, which carried a potential death sentence. Sometime after midnight, he made his way to the railway bridge over the St. Croix River and detonated the dynamite. The bomb damaged the bridge but failed to destroy it. The explosion blew out windows in nearby buildings on both sides of the border, in the hamlets of St. Croix, New Brunswick, and Vanceboro, Maine. The bridge was rendered unsafe but required only minor repairs before trains could cross again. Horn was apprehended, interrogated by the Bureau of Investigation, and signed a confession. He was indicted by a federal grand jury in Boston on March 2, 1915, charged with transporting explosives on a common carrier that also carried passengers. The sentence was eighteen months at the Atlanta Federal Penitentiary. He was by then suffering from advanced syphilis.

The Vanceboro bombing was not an isolated act of desperation but part of a coordinated German campaign to wage covert war on Canadian and Allied supply lines from neutral American soil. Von Papen had orchestrated the operation from his official post as military attache -- a brazenness that contributed to his recall to Germany by the end of 1915. The incident helped awaken American officials to the scope of German espionage activities in the United States, though it would take a far larger attack to fully shock the nation. On July 30, 1916, a massive explosion at the Black Tom munitions depot in New York Harbor destroyed two million pounds of arms and ammunition, shook buildings across Manhattan, and damaged the Statue of Liberty. The Vanceboro bridge bombing, clumsy and ultimately futile, was a rehearsal for that catastrophe -- the first sign that the war in Europe had already arrived on American territory, carried across the border in a suitcase on a sleeping car.