Young McKinley was one of the model soldiers of the regiment. General Hayes said: "We soon found that in business and executive ability he was of unusual and surpassing capacity for a boy of his age. When battles were to be fought, or a service was to be performed in warlike things, he always took his place." McKinley said in after years that he looked back with pleasure upon the fourteen months that he carried a musket in the ranks, for they taught him many things. The regiment was sent into West Virginia, and its first engagement was at Carnifex Ferry. In the summer of 1862 it was ordered to Washington, and a few days after its arrival it joined the Army of the Potomac, which was then moving northward to head off the Army of Northern Virginia, which was bent upon an invasion of the Northern States. The crash of arms came at South Mountain (September 14th) and Antietam (September 17th). At South Mountain the regiment made three successful charges, and lost heavily. Antietam was the bloodiest day of the war, more than 2,000 men on each side were killed on the field, and the Twenty-third Ohio was in the hottest of the fight, holding its position from morning till evening unrelieved. Private McKinley, meanwhile, had been made Commissary Sergeant, and his place was with the supplies in the rear. He pressed a few stragglers into his service and got ready a dinner for the regiment, with hot coffee, and loaded it into two wagons. With these he drove upon the field, under fire. The enemy's shot struck down the mules of one wagon, but with the other he reached his comrades on the firing-line, who gave a great shout of welcome when they saw him. He walked along the line, and fed every man with his own hand. There is no record that such a thing ever was done before or since. For this service he was made a second lieutenant, and in the following February he was promoted to first lieutenant. The regiment was a part of the force that headed off Morgan in his raid into Ohio, fought him at Buffington's Ford, and finally captured him. After that it took part in a series of battles in the mountains and in the Shenandoah Valley. At Cloyd's Mountain, after a wonderful march through ravines and dense woods, they burst into the enemy's camp, McKinley leading his company, which was the first to leap over the fortifications and silence the guns.