When Beckman turned 18 in August 1918, he enlisted in the United States Marines. After three months at marine boot camp on Parris Island, South Carolina, he was sent to the Brooklyn Navy Yard, for transit to the war in Europe. Because of a train delay, another unit embarked in place of Beckman's unit. Then, counted into groups in the barracks, Beckman missed being sent to Russia by one space in line. Instead, Arnold spent Thanksgiving at the local YMCA, where he met 17-year-old Mabel Stone Meinzer, who was helping to serve the meal. Mabel would become his wife. A few days later, the armistice was signed, ending the war.