At the onset of the
American Civil War, Miller's cavalry unit was one of the first militia to offer their services to Pennsylvania Governor
Andrew Gregg Curtin. Miller was mustered into the Union Army as a second lieutenant on August 8, 1861, for a three-year enlistment, and the unit was given the designation Company H of the
3rd Pennsylvania Cavalry. In September 1862, the 3rd Pennsylvania was sent to Maryland to defend against the invasion force of
Robert E. Lee. On September 16, the regiment helped lead Brigadier General
Joseph Hooker's corps across Antietam Creek and Company H was assigned to Hooker's headquarters. The following afternoon, as Confederate Lieutenant General
Stonewall Jackson was pushing towards the Union line, one of his brigades rushed forward on a Union gun battery. Miller came to the aid of the battery and helped rescue guns before they fell to the enemy, and for his gallantry was promoted to captain for his efforts and bravery. assumed a position on the right flank of the Union line near Cress Run which they held for the rest of the day. In the evening, Miller's Company H pushed forward with Company M to slow the advance of the
2nd Virginia Cavalry along the ridge. This prevented the Confederate army from fully fortifying the later attack on
Culp's Hill as it would have left their flank uncovered. in the woods north of the Lott house and assist the
1st New Jersey Cavalry Regiment in repelling Brigadier General
Wade Hampton's advance. With full company agreement, Miller led his men into the flank of the Confederate advance causing confusion among the rebels who believed their retreat path was about to be cut off. and the Union line saved. Later reports credited Miller's decision as one of the turning points of the Battle of Gettysburg, with one reporter even calling him the man who "won Gettysburg". In a 1906 work by
Arthur L. Wagner on military tactics, Wagner contrasted Miller's surprise attack with
Lord Cardigan's
Charge of the Light Brigade in the
Battle of Balaclava. Whereas Cardigan's inaction to attack an exposed flank of the Russian cavalry caused his own brigade to lose the battle, Miller's disobedience served as the catalyst to end the Confederate approach and save the day. After he was mustered out of service, Miller was elected a companion of the Pennsylvania Commandery of the
Military Order of the Loyal Legion of the United States. ==Medal of Honor citation==