On September 17, 1862, two of America’s greatest armies engaged in mortal combat at the Battle of Antietam (or Sharpsburg). Both of these forces were in their infancy. The Army of the Potomac and the Army of Northern Virginia would go on to greater glories on other fields. But perhaps never again would they face so many structural challenges as in the confusing days of September 1862. A comparison of the armies helps to clarify those challenges and identifies the strengths and weaknesses inherent in each command.

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General George B. McClellan was the 35-year-old scion of a noted Pennsylvania medical family with roots dating back to colonial New England and Mayflower. McClellan was well schooled in military matters, ranking second in the famous West Point class of 1846. He had experience both in the Mexican War and as an observer of European armies during the Crimean War. Nevertheless, his frequent caution in combat, coupled with a conservative outlook on how the rebellion should be put down, proved to be his undoing as an army commander. Military successes in western Virginia early in the war brought him favor with the Lincoln administration. This led him back to the seat of government and appointment as commander of the Army of the Potomac, and soon the position of general-in-chief.

McClellan’s Peninsula campaign in the spring of 1862 brought the Union army closer to the Confederate capital in Richmond than any other time until Ulysses S. Grant’s Overland campaign two years later. But McClellan failed to capture his objective. This and ongoing disagreement with the administration cost him his command, but only temporarily. The subsequent defeat of Union Maj. Gen. John Pope’s Army of Virginia in the Second Manassas campaign that August opened the door for McClellan once again. As the Army of Northern Virginia prepared to cross the Potomac into Maryland, Abraham Lincoln was faced with his worst crisis since taking office. McClellan was restored to command and charged with protecting the capital and stopping the Confederates. Within days he organized a new Army of the Potomac in the camps outside Washington.

General Robert E. Lee was a Virginia aristocrat whose lineage included some of the great political and military figures of the early days of the republic. But this is where the similarities between the two commanders quickly diverge. Lee was 55 years old at the time of the Maryland campaign. Unlike McClellan, who in the prewar years had left the Army for lucrative work in the railroad industry, Lee had spent more than 30 years in the Army. During this period he was a cavalry commander, engineer on many of the Atlantic coastal fortifications, superintendent at West Point and a staff officer in the Mexican War. It was in the latter position that Lee gained the valuable experience needed to lead armies in the Civil War. While McClellan often quarreled with the Lincoln administration, Lee had the full support of President Jefferson Davis and the Confederate Congress. Promoted to the rank of full general in August 1861, he took field command of General Joseph E. Johnston’s army after that commander was seriously wounded at the Battle of Seven Pines on May 31, 1862. Leading the Army of Northern Virginia, the new commander drove McClellan from the Peninsula and then launched a lightning campaign into northern Virginia that culminated in the destruction and rout of Pope’s army at Second Manassas. By September 4, 1862, the Confederates were crossing the Potomac into Maryland, in a campaign that would be one of the most desperate of the war for this great army.

The Men
The two armies that fought at Antietam represented a cross section of the American population. The soldiers were primarily from small towns or rural backgrounds. Union regiments claimed more urban enlistments. Around one-fourth of the Union troops were from New York. Pennsylvania was the next largest group. Nearly 25 percent of Lee’s army was from Virginia, with Georgia representing a close second at about 21 percent.

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