Fredericksburg, Virginia, was the site of a major Union defeat in December 1862, with roughly 13,000 Union casualties in a catastrophic assault on well-prepared Confederate positions. The medical response to Fredericksburg was the largest Union field medical operation of the war to that point, mobilizing surgeons and supplies from across the Army of the Potomac and the broader medical corps. Walker worked in this response, in a field environment significantly more demanding than the Patent Office Hospital.
She also worked in the area around Warrenton and Aquia Creek, Virginia — locations in the zone where the Army of the Potomac operated in 1862 and 1863. The specific character of her assignments during this period is not comprehensively documented in a single primary source, and accounts vary in the detail they provide about exactly where she was and what she was doing at specific points.
The move to Chattanooga placed her in the western theater, where the Army of the Cumberland was operating under General George H. Thomas. The Chattanooga campaign of late 1863, which culminated in the Union victories at Lookout Mountain and Missionary Ridge in November 1863, was one of the turning points of the western theater and opened the route for Sherman’s subsequent Georgia campaign. Walker was in Chattanooga during the period following Chickamauga and before the Chattanooga battles, working in the medical infrastructure supporting the besieged Union forces.
It was in this context that the Army of the Cumberland formally hired her in September 1863. The specific motivation — why the Army of the Cumberland hired her when earlier Army commands had refused — is not fully explained in the available records. Thomas’s command may have been more pragmatic about using available qualified personnel. The specific need may have been acute enough to overcome the institutional resistance. Whatever the reason, the contract was issued and Walker finally had formal status.