 Complete the following phrase:  Walker volunteered at the outbreak of American Civil War as a surgeon  first for the Army, but was rejected because she was a woman despite having kept a private practice for many years. She was offered the role of a nurse but declined and chose to volunteer as a surgeon for the Union Army as a civilian. The U.S. Army had no female surgeons, and at first, she was allowed to practice only as a nurse. During this period, she served at the First Battle of Bull Run Manassas, July 21, 1861, and at the Patent Office Hospital in Washington, D.C. She worked as an unpaid field surgeon near the Union front lines, including at the Battle of Fredericksburg and in 