On Sept. 17, 1862, Gen. George B. McClellan stopped Gen. Robert E. Lee’s first Confederate invasion of the North at the Battle of Antietam, the bloodiest day of warfare in American history. This narrow victory changed the course of the war.

Yet history has not been kind to McClellan. Politicians from the 1860s onward and countless historians have claimed he could have easily destroyed Lee’s army during the campaign and ended the war in 1862, sparing the country another two and a half years of bloody conflict.

Their criticism stems from the belief that McClellan moved too slowly and cautiously to attack Lee. They assert that when a copy of Lee’s plans fell into McClellan’s hands, the Union general wasted precious hours before advancing. They declare that McClellan’s forces outnumbered his foe’s by more than two to one and by that metric alone, he should have decimated Lee’s army. They are wrong.

Contrary to what most of the literature will tell you, McClellan was not a hesitant fool. He did his best under challenging conditions.

Scarcely two weeks before the Battle of Antietam, he was a general without a command. He had once held sway over all the Federal armies, but during the previous six months every unit under his control had been transferred to other generals.

An undated photo of Gen. George B. McClellan. (Mathew B. Brady/AP)

Most had been sent to reinforce Gen. John Pope as he fought Lee on the plains of Manassas. Pope, however, was thoroughly defeated, and his demoralized troops streamed back to the capital with the Confederates close behind.

In a moment of desperation, Lincoln returned the shattered remnants of Pope’s army to McClellan, hoping its former commander could reinstill the high morale the troops had possessed a year earlier.

Read the full Washington Post Civil War 150 series.

When McClellan took charge of the Union forces on Sept. 1, he inherited four separate armies, thousands of untrained recruits and numerous other small commands that needed to be made ready in a hurry. To further complicate matters, three of his senior commanders had been ordered relieved of duty, charged with insubordination against Pope.

Acting quickly

Unbeknownst to the Federals, Lee had struck north into Maryland. The cavalry was the arm of the service most likely to discover Lee’s change of direction, but when McClellan took over, there was virtually none available to him.

On paper, McClellan commanded some 28 cavalry regiments. But the disastrous Manassas campaign had worn out the horses of almost half the Union regiments, while most of the remainder were stranded at Hampton Roads by gale-force winds. For the first week of the campaign, McClellan could only count on perhaps 1,500 cavalry from two regiments and a few scattered squadrons from his old army to challenge some 5,000 Confederate cavalry soldiers screening Lee’s army.

Despite these handicaps, in the week it took for Lee’s army to march to Frederick, McClellan’s army traveled an equal distance to redeploy on the north side of Washington. This was accomplished as he reshuffled commands, had his officers under charges reinstated and prepared to fill out his army with untrained recruits.

These new men, organized into 1,000-man regiments, would account for about a fifth of McClellan’s force at Antietam. Northern recruiting booths had only reopened in July, and the first of these regiments were not assembled in their home states until mid-August. Before officers learned how to issue orders or their men learned to follow them, they were sent by train to Washington and immediately marched to the front. They would learn how to fire a musket as they marched to battle.

In the second week of the campaign, Lee’s army suddenly left Frederick and marched west.

As McClellan’s army advanced on Sept. 13, Union soldiers stumbled upon a four-day-old copy of Lee’s orders in an abandoned rebel camp. Known as Special Order No. 191, this paper revealed that Lee had dangerously split his army into five parts. Three columns had converged on Harpers Ferry to capture the Federal garrison there, a fourth column was in Hagerstown, and a fifth column was acting as a rear guard near Boonesboro, Md. Historians have debated fiercely over when the Lost Order was delivered to McClellan.

In his landmark 1983 book, “Landscape Turned Red,” Stephen Sears asserts that McClellan verified before noon that the papers were legitimate, then exhibited his usual excessive caution and failed to move his army for 18 hours. To back up this theory, Sears cites a telegram that McClellan sent to Abraham Lincoln at “12 M” — which Sears says stands for meridian or noon — in which McClellan confidently informs the president that he has the plans of the enemy and that “no time shall be lost” in attacking Lee.

After the book’s publication, though, the original telegram receipt was discovered by researcher Maurice D’Aoust in the Lincoln papers at the Library of Congress. It shows that the telegram was sent at midnight (the word was written out) — a full 12 hours later than Sears thought. D’Aoust points this out in the October 2012 issue of Civil War Times in an article entitled “ ‘Little Mac’ Did Not Dawdle.”

The sequence of events most likely went like this: The Lost Orders were found “about noon,” as confirmed by the unit commander, and reached McClellan shortly before 3 p.m., which is when he ordered his cavalry chief to verify that the paper was legitimate, and not some ruse planted by the rebels. Even before the orders could be verified, McClellan had the vanguard of the army, Burnside’s 9th Corps, on the move at 3:30 p.m. These men filled the road west to Lee’s rear guard at South Mountain well into the night. Near sundown, at 6:20 p.m., he began to issue orders for the rest of his army to move, with most units instructed to be marching at sunrise. (They were roused from sleep at 3 a.m.) In the midst of this activity, at midnight, the general telegraphed the president to tell him what was going on.

No dilly-dallying there.

By 9 a.m. on Sept. 14, the first troops had climbed South Mountain and met the Confederate rear-guard in battle. By nightfall, McClellan’s army carried the heights and forced a defeated Lee to find a new defensive position along Antietam Creek. McClellan pursued the next morning and within 48 hours initiated the Battle of Antietam, which forced Lee back across the Potomac River.

Underestimating the damage?

In his after-action report, McClellan claimed that his men buried 2,700 Confederates on the Antietam battlefield and captured 6,000 more. He could only guess at the number of wounded, but he estimated it was 18,742 men, using the ratio of killed to wounded for his own troops.

This stands in stark contrast to Confederate reports, which claimed losses of 1,674 dead, 2,292 missing and 9,451 wounded — a total of 13,417. Even discounting the wounded, the discrepancy between the two reports is almost 5,000 casualties.

Which is right? The burial grounds would indicate that McClellan’s number is closer to the truth. More than 3,300 dead rebels specifically associated with the Antietam campaign can be found buried in the Confederate cemeteries in Hagerstown, Frederick, Shepherdstown and Winchester. This number is larger than McClellan’s because it includes bodies buried by the Confederates themselves as well as those who died shortly after the battle.

As for the captured Confederates, McClellan’s medical director, Jonathan Letterman, reported 2,500 wounded under his care following the fight. At least another 2,500 unwounded prisoners of war were transferred from the battlefield to Forts Delaware and McHenry, bringing the number of captured rebels to more than 5,000 — much closer to McClellan’s figure than Lee’s. This would make what is already America’s bloodiest day even more horrific than previously thought, and it would mean McClellan did more damage than he is credited with.

Underestimated strength

Perhaps the most important misconception is the number of troops Lee brought with him during his invasion. Most historians cite McClellan as having had 87,000 men and Lee around 40,000. These numbers are often used for the entire three-week campaign, with the Confederates sometimes credited with as many as 55,000 men, 15,000 of whom straggled off before the battle. But there are no complete returns for Lee’s army until Oct. 10, 1862. Every historian’s count is merely a best-guess estimate.

Lee filed his first return five days after the battle, noting the count is “very imperfect” and does not include cavalry or artillery. It states that on Sept. 22, he had at least 36,418 infantry. Adding a conservative number of 5,000 for the missing cavalry and artillery units would bring his total to about 41,000 troops at the end of the campaign.

Eighteen days later, on Oct. 10, Lee filed his first complete report, which showed 64,273 present for duty. This number is significant because Lee had not received a single new regiment to replace his losses; nor did he receive many, if any, recruits because the February draft law had already pulled every eligible man into the army by early summer.

If we add Lee’s reported campaign losses of 13,417 (which, as already noted, are too low), it would show that Lee started the campaign with at least 75,000 men.

Most historians will explain this away by citing the Confederate claim that almost half of Lee’s army — 30,000 soldiers — straggled behind. Where is the corroborating evidence? The Official Records show that some 5,000 rebels moved to Winchester at the start of the campaign, then on to Lee’s army after Antietam, but what about the rest? How could any rebel straggle in Maryland — as many Confederates claimed — and not be captured by the Union army, which immediately occupied every post the retreating Confederates vacated? If the straggling took place in Virginia at the start of the campaign, who fed these 25,000-plus soldiers? Who led them? How did they all get back into Lee’s army so quickly through countryside most had never been in?

The simple answer is that that Confederates had suffered a major loss and needed some way to explain it. While straggling undoubtedly occurred in the last few days before Antietam, 30,000 men were not missing for most of the campaign.

Plenty of eyewitness accounts support the 75,000 figure for Lee’s army. Perhaps the most detailed comes from Dr. Lewis Steiner of the Sanitary Commission, who happened to be in Frederick on Sept. 10-11 as most of the Confederate army marched out of town. Steiner tried to count every rebel that passed him and concluded by the end of the two days that he had seen some 69,000 Confederates. However, he did not witness any cavalry or a division south of town that was also part of Lee’s army. When the most conservative estimates for these troops are added to Steiner’s numbers, they bring the total to well over 75,000.

So much for McClellan’s outsized numerical advantage. The army he drove back was not much smaller than his own. He did it without proper cavalry support, with his superiors hoping to oust him and with a significant portion of his army untrained. And as it turns out, he inflicted more damage on Lee’s army than anyone suspected.

Related article, In defense of McClellan: A contrarian view - the Peninsula campaign