In 1915, Straight left J.P. Morgan and went to work as a vice-president for American International Corporation. In that same year, Straight became involved with the Preparedness Movement and attended the July 1915 Citizens' Military Training Camp in Plattsburgh, New York. When the United States entered World War I two years later, Straight joined the United States Army. He served stateside and later in France with the Adjutant General's Corps and First Army. For his service, he was awarded the Distinguished Service Medal and served as a major.