A few days after the war began, Horn received orders to report to Germany for military service. Being a retired captain, he pursued these orders by closing his business affairs in Guatemala, where he managed a coffee plantation, and went to Galveston, Texas, where he tried to get passage to Germany. However, no steam ship line would take him.
He arrived in Vanceboro late on the afternoon of January 31st and at 7 pm that night, by previous instruction, he met an Irish man that he had never met before. After giving the password, “Lo Tommie”, the man gave Horn a satchel containing 80 pounds of nitro glycerine. He took this satchel to his room at the Teague Hotel and there it stayed until Monday night. February 2nd, when he embarked on his mission which startled all of New England and the Maritimes.
Shortly before 8 pm on Monday night, Horn went to the bridge in wickedly cold weather, with his nitro glycerine under his arm. Though he could have set off the explosive at any time, he wanted to make sure that no fatalities would follow the destruction of the bridge. So he waited until the block signals told him there was no train within a mile. He spent hours in nearby woods waiting for the right moment.
But finally the work was done and he left for the hotel while the three minute fuse slowly counted down until a tremendous explosion shook the earth and shattered scores of windows in Vanceboro.
Numbed and frozen by the cruel hours above the bridge that resulted in both of his ears chilled and his thumb frozen, he dared not attempt escape and was arrested a few hours later in his room at the Teague Hotel. He was taken to Bangor on the charge of breaking windows and was held for 30 days. Later he was charged for the illegal transportation of nitro glycerine across state lines. Despite his bitter fight for freedom, the American federal authorities said he was guilty of a criminal act, not act of war, and like any other criminal he would have to suffer the consequences. He spent several years in various federal prisons until he was deported to Germany. His fate after returning to Germany remains unknown.