The Army of the Cumberland, commanded at this stage by General George H. Thomas, was operating in Tennessee and Georgia, and Walker was assigned to work with the 52nd Ohio Infantry. The 52nd Ohio was part of the force operating in the same general region as the Battle of Chickamauga, one of the bloodiest engagements of the entire war, fought September 19-20, 1863, with roughly 34,600 combined Union and Confederate casualties.
Her specific duties included treating both soldiers and local civilians. The practice of treating civilians — crossing the lines between the armies to reach people who were caught in the contested territory of the conflict — was the behavior that would eventually lead to her capture. It was also, by the standards of her later reputation, the most distinctly humanitarian dimension of her service: a physician going where the people who needed medicine were, regardless of which army’s lines they happened to be behind.
Whether Walker was also gathering intelligence during these crossings is a question the surviving records do not definitively answer. She was known to report on Confederate troop movements and positions after returning from forays into Confederate-held territory. General William T. Sherman is said to have valued this intelligence. But the line between observation during medical crossings and deliberate espionage was not cleanly drawn, and the Confederate forces who eventually captured her drew the same ambiguous conclusion the historical record does.
## Castle Thunder: Prisoner of War
On April 10, 1864, Walker was captured by Confederate forces shortly after she had assisted a Confederate surgeon with an amputation. The specific circumstances suggest she had crossed into Confederate-held territory, as she had done before, and that the Confederate forces had made their own assessment of the situation.