When the Civil War broke out, she decided to contribute to the war effort by gathering and distributing needed supplies to wounded soldiers. At war's end, she compiled records to assist in the identification of missing troops.
She was in Europe when the Franco-Prussian War began in 1870 and again she devoted herself to helping the wounded. At this time she became associated with the International Red Cross. Upon her return to the United States in 1873 she began a campaign to create a branch of that organization in her home country. In 1881-1882 she was elected the first president of the newly-formed American Red Cross and convinced the U.S. government to accept the 1864 Geneva convention about treatment of enemy prisoners. She was also the first to amend the charter of her new organization to allow it to provide relief for all disasters, both natural and political.
She directed relief efforts to cope with disasters throughout the 1880's and 1890's. Her work in the Spanish-American war was often tarnished by splits in the organization she founded and the refusal of many generals to permit her access to the wounded. Many criticized her because of her age and authoritarian behavior. President McKinley, a devoted admirer of Barton and the American Red Cross, permitted the incorporation of the society in 1900.