Miller received his country's highest award for bravery during combat, the Medal of Honor, for actions taken on July 3, 1863, during the Battle of Gettysburg.

Said to have saved Gettysburg for his decision to break rank, this was claimed to be the first time in American military history where a soldier was awarded for disobeying a direct order.

[3] 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.

[2] During this campaign, Miller met with Prince Philippe, the Count of Paris, who was serving as assistant adjutant general to McClellan with the rank of captain.

Miller's company, totaling roughly 100 troops,[5] assumed a position on the right flank of the Union line near Cress Run which they held for the rest of the day.

[6] On July 3, Miller was given orders to hold his company "at all hazards"[9] in the woods north of the Lott house and assist the 1st New Jersey Cavalry Regiment in repelling Brigadier General Wade Hampton's advance.

[5] When the enemy brigade advanced, Miller noticed a large contingent led by Confederate Brigadier General Fitzhugh Lee threatening to break through a main section of the Union line.

"[8][10] 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.

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.

[4][17] 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.

Without orders, Captain Miller led a charge of his squadron upon the flank of the enemy, checked his attack, and cut off and dispersed the rear of his column.