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Patrick Semansky, Associated Press
Jim Rosebrock, a volunteer portraying a member of the Union Army's 4th U.S. Artillery, prepares for a cannon firing demonstration at Antietam National Battlefield in Sharpsburg, Md., Monday, Sept. 17, 2012. A series of demonstrations and speeches took place at the battlefield to commemorate the 150th anniversary of the Civil War's Battle of Antietam.

One hundred and fifty years ago this week, America experienced its bloodiest single day as Union and Confederate armies met on the battlefield near Antietam Creek. The Sept. 17, 1862, Battle of Antietam (or Sharpsburg, as the engagement came to be known in the South) was the culmination of Confederate Gen. Robert E. Lee's invasion of Maryland and arguably represented the high water mark for the Southern cause in the Civil War both militarily and politically.

Flush with victory over the Union armies during the Peninsular Campaign and the Battle of Second Bull Run in Virginia, Lee felt the time had come to wage an offensive war against the North. The decision for the invasion of Maryland in early September 1862 was twofold: first, though Maryland had remained loyal to the Union, the state was still a slave-holding state with considerable Southern sympathies. It was hoped that Lee's army could rally pro-Southern elements and the state would defect to the Southern cause.

Secondly, and much more important, both Confederate President Jefferson Davis and United States President Abraham Lincoln were keenly aware of the power and influence of the European powers. Lincoln desperately wanted England and France to remain neutral, while Davis hoped the Europeans would officially recognize the Confederacy and force arbitration upon the North, effectively guaranteeing Southern independence. A Confederate victory in the North could go a long way to persuade the British to support the South.

The English were deeply divided about the Confederacy. On one hand, most Englishmen felt that the Southern states had a right to secede and desperately needed Southern cotton to keep British textile mills in operation. On the other hand, most English despised the practice of slavery, which had been banned throughout the empire in 1833.

Still, the prospect of British recognition of the South was very real. At one point William Gladstone, at this time chancellor of the Exchequer, made a speech in which he said, “We may have our own opinions about slavery, we may be for or against the South, but there is no doubt that Jefferson Davis and other leaders of the South have made an Army; they are making, it appears, a Navy; and they have made what is more than either, they have made a nation.”

In Washington, Lincoln was beset by abolitionists urging him to make some kind of declaration outlawing slavery. Lincoln, however, still believed the Constitution prevented him from arbitrarily ending the practice. Finally, one legal argument was made that suited Lincoln's desire to end slavery.

In his book, “Antietam: Crossroads of Freedom,” historian James McPherson writes: “Thousands of slaves worked as army laborers, teamsters, cooks, musicians, servants and in other support capacities. They provided the logistical 'tail' of these armies ... and thereby freed a high proportion of Confederate soldiers for combat duty. As time passed, more and more Yankees began asking: Why not convert this Southern asset of black labor into a Northern asset by confiscating slaves as enemy property, freeing them and putting them to work for the Union?”

Lincoln had his legal basis for ending slavery — it could be abolished as a war measure. Fearing that such a declaration on the heels of the defeats in Virginia would only look like a move of desperation, Secretary of State William Seward advised Lincoln to wait until after the next major victory over the Confederacy. The Emancipation Proclamation, he argued, must be delivered from a position of strength.

Meanwhile, Lee's 55,000-man army crisscrossed Maryland in a grim game of hide and seek with Union Gen. George B. McClellan's 75,000-man Army of the Potomac. It was then that chance intervened.

In “The American Civil War: A Military History,” author Sir John Keegan writes: “A strange episode then compromised (Lee's) strategy. Lee's plans, set out in a special order, No. 191, detailing the separate movements of of his army, were found by a Union soldier wrapped around three cigars in an abandoned Confederate camp. The paper was taken to McClellan's assistant adjutant general ... Even the ever-timorous McClellan was persuaded that he had been granted the most extraordinary stroke of good fortune.”

McClellan positioned himself in Lee's path, and the two armies clashed in one of the most brutal engagements of the war. The tight conditions of the battlefield meant that both armies occupied a relatively small geographic area, leading to intense, heavy fighting and extraordinarily high casualties on both sides. Fearing incorrectly that Lee had more men in reserve and unwilling to commit his own reserves, McClellan never achieved local troop concentration. Though defeated, Lee's army was able to escape back to Virginia. If McClellan had been less cautious the rebellion could have ended nearly three years earlier than it did.

Keegan writes: “As the dreadful day drew out, the number of dead and wounded mounted. The eventual total was 12,400 casualties on the Union side, 10,300 on the Confederate.”

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With Lee's failure in Maryland, Lincoln had his victory and issued the Emancipation Proclamation five days later. The British population wildly approved the measure, and without popular support, the British government never again seriously considered recognizing Confederate independence. The weeks leading up to the Battle of Antietam were the closest the South ever came to European intervention in the Civil War.

As a final note, one of the sources for this story, Keegan, passed away on Aug. 2 at his home in England. A celebrated military historian, Keegan authored numerous works over the course of a marvelous career, and his unique perspectives and narrative abilities will be sorely missed. In addition to his book on the Civil War, I highly recommend his histories, “The First World War,” “The Face of Battle,” “The Mask of Command” and “A History of Warfare.”

Cody K. Carlson holds a master's degree in history from the University of Utah and currently teaches at Salt Lake Community College. He is also the co-developer of the popular History Challenge iPhone/iPad apps. Email: ckcarlson76@gmail.com