With the conclusion of the Civil War in June 1865, MacArthur resigned his commission and began the study of law. After just a few months, however, he decided this was not a good fit for him, so he resumed his career with the Army. He was recommissioned on February 23, 1866, as a second lieutenant in the Regular Army's 17th Infantry Regiment, with a promotion the following day to first lieutenant. Because of his outstanding record of performance during the Civil War, he was promoted in September of that year to captain. However, he would remain a captain for the following two decades, as promotion was slow in the small peacetime army.