Her work as a Civil War nurse and relief worker began with the Baltimore Riot

Four Massachusetts troops were killed when Confederate sympathizers attacked them on April 19, 1861, barely weeks after the start of the Civil War. Near Barton's workplace at the U.S. Patent Office, the incomplete U.S. Capitol building received the injured.

When Barton discovered that several of the men were her former students, she was horrified and hurried to save the injured. She remarked, "They were faithful to me in their boyhood and faithful to their country in their manhood. The city lacked supplies and order, and it had not been adequately ready for the inflow of soldiers. Barton immediately got to work gathering donations, which eventually filled multiple warehouses. She then started taking her supplies to battle and attending to the wounded soldiers herself.

She swiftly grabbed supplies from her home, including food, medication, and clothing, and assisted with their care. It marked the start of Barton's nursing career during the Civil War, earning her the moniker "Angel of the Battlefield."