A week after the Civil War started on April 12, 1861, southern sympathizers attacked soldiers in the 6th Massachusetts Infantry. The wounded soldiers were taken to the capitol building, which was being used as a temporary hospital for them. Clara wanted to help them, so she brought supplies over. She got permission to travel with the Union Army to help people on the battlefield in 1862.
During the Civil War, Clara traveled with the Union Army, and helped take care of the wounded soldiers on the battlefield. She brought supplies in supply wagons for them, and saved many lives. She was called the "Angel of the Battlefield."
In 1869, Clara went to Europe to rest. While she was there, she learned about the International Red Cross in Geneva, Switzerland. There was an international agreement about rules about caring for the sick and injured soldiers during wartime. It was called the Geneva Treaty. When she came back to America, she tried to get people's support to join the Geneva Treaty, and President Chester A. Arthur signed the treaty in 1882. The American Red Cross was created.