Now it was the South’s turn to take the offensive. After Union General McClellan failed to take Richmond, Confederate officials plotted to recapture lost territory in the West and threaten Washington D.C. by a campaign in the East.  While armies under Braxton Bragg and Edmund Kirby-Smith marched into Tennessee and Kentucky, Lee marched into Maryland where he hoped to achieve two things—seize the railroads feeding Washington D.C., thereby isolating the city, and rally pro-Southern inhabitants of the border state.  After Maryland, Lee intended to march into Pennsylvania.  Suspecting that the Northerners’ pain threshold was low, he planned to take the war to them and force pressure on Lincoln to abandon the conflict.

Lee’s march through Maryland advanced quickly.  But as he approached the Pennsylvania border he decided that he needed to pause to secure his line of communication with Virginia. He therefore split off a portion of his army (some 11,000 men) to take Harper’s Ferry.  It was a safe move, given all that Lee knew about his overly cautious opponent.  McClellan, who had recently brought his army back from the James River, would probably never find out that Lee’s forces were divided—and even if he did, he would be slow to react.

But for a brief moment, McClellan’s stars aligned.  A Southern courier lost a copy of Lee’s marching orders; Union soldiers found them wrapped around a couple of cigars.  And with this intelligence, even the slow-moving McClellan was able to catch a large a portion Lee’s army camped near Sharpsburg, Maryland. 

McClellan was able to bring a huge numerical advantage against the divided Southern force (75,000 to 40,000).  But still, he began the attack by cautiously probing the Confederate lines. Then after mounting a small diversionary attack on Lee’s right, he concentrated his forces for a massive attack on Lee’s left flank.  But Lee sniffed out McClellan’s plan, and proved far more effective than the Northern general in shifting his outmanned army to meet the brunt of the Union advance.

The fighting was horrific—the single day’s carnage was unprecedented.  By evening more than 5000 had been killed and another 20,000 wounded.  At one point, Union troops had managed to break through the Confederates’ first line of defense that paralleled a sunken road—Bloody Lane it was labeled.  But reluctant to commit his reserves, McClellan was unable turn this breakthrough into a more complete victory.  And the day ended with neither side gaining a significant advantage.

Lee expected McClellan to resume the attack the next morning; when he didn’t, Lee began moving his army out, southward toward Virginia. 

The battle thus ended more or less in a draw with a slight advantage to the North. But the overall ambiguity of the results far better served the North than the South.

Great Britain had been debating whether to recognize the Confederacy.  Its huge textile industries were dependent on the South’s cotton, and Confederate diplomats had skillfully worked this dependence toward British recognition of their government.  Recognition would lead to access to British industries—guns, munitions, and other needed supplies.  And if Britain traded with the South, it would protect this trade with its massive navy.  In short, British recognition would level the industrial and naval playing field. 

But the ambiguous results at Antietam put a halt to these discussions.  A British cabinet meeting set to finalize British plans was cancelled once word of Lee’s withdrawal from Maryland was received in London.

Moreover, for the North, Antietam gave Lincoln the “victory” he needed to issue his Emancipation Proclamation.  For some time he had wanted to announce the ultimatum that would give Southern states 100 days to abandon their rebellion or risk losing their slaves. Without some sort of victory, this announcement would look desperate or, even worse, comical. But Antietam gave Lincoln’s threat more credibility.  Consequently, just six days after the battle, Lincoln issued his preliminary proclamation which declared that slaves would be freed on 1 January 1863 in those states still in rebellion.

Antietam changed the course and the meaning of the war. The fate of the Confederacy and the cause for which the North fought, turned on a lost message and some unsmoked cigars.