Harrow briefly resigned from the army in July 1862 after being accused of drunkenness at Kernstown, but was reinstated to command the next month. Serving in the Army of the Potomac at Antietam, Harrow's regiment attacked enemy positions along the Sunken Road, where half his men became casualties. Harrow's report praised all his men for their performance during four hours under heavy fire in the battle. As senior colonel in his brigade, Harrow later became its commander and led it through the Fredericksburg Campaign. In October 1862, Kimball had sent a letter to President Lincoln personally requesting that Harrow be promoted to brigadier general. 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.