[19] McKinley spent the winter substituting for a commissary sergeant who was ill, and in April 1862 he was promoted to that rank.[20] The regiment resumed its advance that spring with Hayes in command (Scammon led the brigade) and fought several minor engagements against the rebel forces.[21]
That September, McKinley's regiment was called east to reinforce General John Pope's Army of Virginia at the Second Battle of Bull Run.[22] Delayed in passing through Washington, D.C., the 23rd Ohio did not arrive in time for the battle but joined the Army of the Potomac as it hurried north to cut off Robert E. Lee's Army of Northern Virginia as it advanced into Maryland.[22] The 23rd was the first regiment to encounter the Confederates at the Battle of South Mountain on September 14.[23] After severe losses, Union forces drove back the Confederates and continued to Sharpsburg, Maryland, where they engaged Lee's army at the Battle of Antietam, one of the bloodiest battles of the war.[24] The 23rd was in the thick of the fighting at Antietam, and McKinley came under heavy fire when bringing rations to the men on the line.