Civil War Union Army Officer. He enlisted at the start of the Civil War as a Private in the regiment which was to become the 2nd Pennsylvania Reserves. Made its Major after the unit was reorganized in August 1861, he was promoted to Lieutenant Colonel prior to the Spring 1862 Peninsular Campaign. He fought in the Battles of Mechanicsville and Gaines Mill, and received particular praise from Brigadier General George A. McCall, his division's commander. Promoted to Colonel and commander of the regiment in August 1862, he was wounded during the Second Battle of Bull Run. His wound forced him to miss the Antietam Campaign, but he was back in command during the Battle of Fredericksburg, where his unit was one of the few Union regiments to breach the Confederate lines. He assumed command of his brigade during that battle when its commander was wounded, and would remain its leader for over a year and a half. During the 2nd Day of the Battle of Gettysburg (July 2, 1863) his command was positioned on the northeast slope of Little Round Top, waiting in reserve. When the Regular Army units of Colonels Sidney Burbank and Hannibal Day were pushed back from the Wheatfield area by Rebels of Lieutenant General James Longstreet's Corps, Colonel McCandless led a counterattack that forced the oncoming Confederates back through the Wheatfield and Rose Woods. This was in effect the last of the fighting in that area of the Battlefield, and his command spent the night against a stone wall on the east side of the Wheatfield. During the fighting the next day, the Pennsylvania Reserves thrust back into the Wheatfield after Pickett's Charge was driven off, and recaptured the bloody ground, as well as over 200 Confederate soldiers. Colonel McCandless would remain in command of the brigade - and often times the division when commander Brigadier General Samuel W. Crawford was absent - until he was wounded in the arm at the Battle of Spotsylvania. That wounding cost him his arm and ended his military service. During his time in the Union Army he was repeatedly commended by superiors, and his Corps commander, Major General Gouverneur K. Warren, tried hard to have him advanced in rank; he received no promotions either by commission or brevet, despite a stellar battle record.