During the American Civil War, Hamilton entered the Union Army as a private in 1861 and served in Company A, Fifth Regiment, Pennsylvania Reserves; appointed judge advocate of the general court-martial and general pass officer for the Army of the Potomac; served on the staff of the Military Governor of Washington, D.C., until transferred to Marianna, Florida in 1865.
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His 3rd Division, the Pennsylvania Reserves, led by Brig. Gen. Samuel W. Crawford, attacked from Little Round Top, drove the Confederates across the "Valley of Death" and ended the deadly fighting in the Wheatfield.
The last combat on the Battle of Gettysburg, Third Day, was "in the early evening. Colonel William McCandless's brigade of Pennsylvania Reserves advanced across the Wheatfield into Rose's Woods where they managed to inflict heavy losses on the 15th Georgia" which had failed to retreat to Warfield Ridge after Longstreet's Assault.
Bucktails, the name of a political faction in New York State or the 13th Pennsylvania Reserves, an American Civil War unit
His Medal of Honor was officially awarded on July 17, 1866, and he is one of only two recipients of the medal from the 11th Pennsylvania Reserves, the other being Henderson C. Howard.