Antietam, the bloodiest single-day fight in American military history, demonstrated that the Union army could defeat the Confederate army in the Eastern theater. It also provided President Abraham Lincoln the courage to release the preliminary Emancipation Proclamation when he was at his strongest, rather than when he was at his weakest. The battle also marked the beginning of the end for General Robert E. Lee's hopes for victory on Southern soil.
The total number of casualties on both sides of the conflict is unknown. Estimates range from 10,000 to 20,000 dead bodies lying in fields near the battlefield of Antietam. By comparison, there were 8,000 killed and 15,000 wounded at Gettysburg one year later.
It is because of these high numbers that many believe Antietam to be the bloodiest battle of the Civil War.
During the war, other battles were more violent, but they did not include as many deaths. For example, at Shiloh (April 6-7, 1862) approximately 7,800 men were engaged, with a loss of about 18,000 all together. At Fredericksburg (December 11-15, 1862) around 4,500 soldiers died, plus another 2,200 prisoners. At Stones River (January 31-February 1, 1863) about 6,100 people lost their lives, while at Chickamauga (September 19-20, 1863) 3,600 soldiers lost their lives.
The thunder of war began to diminish after twelve hours of fighting. Most crucially, Union victory at Antietam gave President Abraham Lincoln the chance he had sought to issue the Emancipation Proclamation, cementing the Battle of Antietam as a pivotal moment in the American Civil War. The proclamation, which abolished slavery in those states that were still in rebellion against the United States, was signed by Lincoln on September 22, 1862.
On September 17, 1862, General John Pope received orders from Secretary of War Edwin M. Stanton to move his army out of Washington, D.C. and into Maryland. His mission was to protect the capital from Confederate attacks while also providing support to Major General George B. McClellan's Army of the Potomac if it needed assistance. By early October, McClellan's army had arrived in Maryland and set up its headquarters just thirty miles outside of Baltimore. On October 7, 1862, Confederate forces under General Robert E. Lee attacked McClellan's army near Antietam Creek in western Maryland. The battle lasted for six hours and cost both sides about 2300 men; however, McClellan withdrew back across the Potomac River into Virginia with only partial success. News of Antietam reached Washington, D.C. on October 16, 1862; Lincoln called it "a great victory" which helped him overcome political difficulties he was facing at the time.
Victory is declared by the Union. Military historians regard the Battle of Antietam as a draw. Nonetheless, the Union declared triumph. Keeping the Confederates in their southern box also allowed President Lincoln to issue his Emancipation Proclamation on September 22, 1862. The proclamation abolished slavery in areas under Confederate control. It was designed to increase pressure for secessionist states to end slavery.
The battle was fought on Maryland's Eastern Shore between McClellan's Army of the Potomac and Lee's Army of Northern Virginia. It was the largest single-day battle up to that time with estimates of up to 20,000 soldiers involved.
McClellan wanted more than just a victory at Antietam, he wanted a complete military defeat of the Confederacy. His plan was to attack the Confederate army while it was still near Washington, D.C., then use its strength to overrun the South. But Lee had already started moving his army away from Washington when McClellan attacked him at Antietam. Although defeated, the Confederate army only lost about 2,100 men at Antietam while the Union army lost about 12,500 people - including McClellan himself.
After Antietam, Lee decided not to fight any more battles outside of Virginia because he did not want to lose more men.
The victory at Antietam provided Abraham Lincoln with a platform from which to deliver the proclamation. This altered the nature of the Civil War and barred European nations from interfering with the actions of its commercial partners in the southern Confederacy. It also demonstrated that the North could beat the South on battlefield ground.
Lincoln had been reluctant to issue the preliminary Emancipation Proclamation because he was worried it would be seen as provocative by the European powers. The battle changed all this; it showed that the North could fight and win a war on its own terms. Now that Britain and France were involved, there was no going back: the war needed to be ended either by military success or through negotiation.
This is shown by two events that followed Antietam battle. First, when Robert E. Lee invaded Maryland later that year, Lincoln issued a new Emancipation Proclamation. This time, it was made permanent. Second, when Union general George McClellan failed to advance further into Virginia in 1864, the president replaced him with William T. Sherman. This was because Lincoln wanted someone who would do whatever was necessary to end the war quickly. He didn't want to be bogged down in negotiations for peace agreements like the one that lasted between 1864 and 1865.