Final answer:
Clara Barton founded the Red Cross after serving as a nurse during the Civil War. She established the American Red Cross in 1881, marking a major step in humanitarian efforts and wartime medical care.
Step-by-step explanation:
The individual who founded the Red Cross after serving as a nurse in Union battlefield units during the Civil War is A. Clara Barton. Clara Barton provided critical nursing care during the Civil War and later went on to establish the American Red Cross in 1881, which became a major humanitarian organization offering aid in wartime and peacetime disasters.
Women such as Barton played pivotal roles during wartime, often volunteering as nurses, ambulance drivers, or engaging in other medical support through the Red Cross. Their dedication provided vital care for the injured and significantly contributed to war efforts, with their legacy continuing to impact medical and humanitarian aid practices.