Lincoln signed the letter on October 16 and forwarded it to the War Department. However, no action was taken and Harrow remained a colonel through the balance of the winter. In April 1863, Harrow was finally promoted to brigadier general and led the 1st Brigade, 2nd Division, II Corps, during the Gettysburg Campaign. On July 2 at the Battle of Gettysburg elements of his brigade were engaged against the attack of Maj. Gen. Richard H. Anderson on the Union lines. Most notably the 1st Minnesota Volunteer Infantry distinguished itself in stalling a Confederate attack on an under-defended part of the Federal line. Harrow served as acting division commander while Brig. Gen. John Gibbon led the corps late in the day. Maj. Gen. Winfield S. Hancock at that time was in charge of the left flank of the army. Defending a portion of the Union line on Cemetery Ridge during the climatic fighting on July 3, Harrow's men helped repel a part of Pickett's Charge. With the wounding of Gibbon, Harrow was elevated once more to command of the 2nd Division. The division had suffered over 1,600 casualties out of 3,773 men in a contest that Harrow reported had raged with "unparalleled ferocity." Harrow's men captured four Confederate battle flags in their front. His own brigade suffered as many as 768 casualties. (Harrow reported 722 losses.) However, nagging questions persisted about Harrow's ability to work cooperatively with his superiors and his abilities to lead and inspire soldiers, as well as about his sobriety. In his official report on Gettysburg, John Gibbon, Harrow's superior, openly praised his other two division commanders, but, notably, did not mention Harrow in his litany of officers deserving recognition. Soon afterward, Harrow was relieved of command and never again served in the Army of the Potomac, but he was soon reinstated in field command by his old friend, President Abraham Lincoln. Harrow was reassigned to the Western Theater, where he participated in the Atlanta Campaign in command of the 4th Division of the XV Corps, a change regretted by those soldiers. His men initially resented him, as "Western officers and soldiers held a sort of prejudice against the Eastern army," in which Harrow had most recently served. Compounding the problem, "General Harrow was far from possessing the necessary qualifications to induce men to obey him ..." and instituted a series of strict and unusually harsh disciplinary measures.