 When the Civil War began in 1861, he was appointed surgeon of the 14th Illinois Volunteer Infantry Regiment and eventually achieved the rank of major.Following his honorable discharge in 1864, Stephenson began practicing medicine in Springfield, Illinois, and soon began plans for a national association of Union veterans.He named the new organization the Grand Army of the Republic (GAR), wrote the GAR's constitution, and adopted the GAR motto "Fraternity, Charity, and Loyalty".