New Brunswick NJ bm.  At the outbreak of the Civil War, he was commissioned a second lieutenant in the 2nd U.S. Cavalry. He was promoted to first lieutenant in April 1862 and served in the Seven Days Battles and the Battle of Antietam, receiving promotion to major between the battles. McIntosh was commissioned colonel of the 3rd Pennsylvania Cavalry on November 15, 1862. In that role he rose to brigade command in the Cavalry Corps of the Army of the Potomac. He led his brigade in the campaign culminating in the Battle of Chancellorsville, winning plaudits from division commander Brig. Gen. William W. Averell.When Maj. Gen. Alfred Pleasonton reorganized the Cavalry Corps following the Battle of Brandy Station, McIntosh became a brigade commander in the second division led by Brig. Gen. David McM. Gregg. McIntosh was ill after Chancellorsville, but he was present when Gregg's division fought at the Battle of Gettysburg. He distinguished himself in the fight against J.E.B. Stuart on East Cavalry Field on July 3, 1863. When a Confederate attack led by Maj. Gen. Wade Hampton was at its height, McIntosh led some of his men in a flank attack on the attacking troopers.[1] McIntosh was injured by a fall from a horse in September 1863; and, after recovering from his injury, he was on duty in the defenses of Washington, D.C., in XXII Corps until May 1864.