McClellan stopped Lee's invasion of the North at Antietam. So what?

source

Over one hundred fifty years ago, two armies gathered on either side of  the Antietam creek in Maryland. In what would be the bloodiest day of the bloodiest war in United States history, more than twenty thousand men were lost in what proved to be an indecisive battle. So what?

Having staved off multiple invasions of the Confederate States of America, General Robert E. Lee, in command of the Army of Northern Virginia, decided to seek an end to the American civil war by invading Union territory and further degrading the enemy's morale. In early September, the Confederate army entered Maryland seeking a conclusive victory over the Army of the Potomac. After General Thomas "Stonewall" Jackson captured Harper's Ferry from the Union troops, the main body of Lee's men met the northern army under General George McClellan at the Antietam creek near Sharpsburg, Maryland. On September 17, 1862, the two armies engaged in a day that would reverberate through history. Though the action that day proved indecisive, the Union army repulsed the Confederate advance and ended the invasion of Maryland. Following the Battle of Antietam, Lee returned to Virginia to continue the war on his home soil.

I love military history, but I wouldn't want to bore you with the details of the battle (may I recommend the Wikipedia page on the battle if you're interested). Instead, I would like to focus on a few key points that show how a taken or missed opportunity can change the course of history.

A few days before the battle, a detail of Union soldiers found written orders drawn up by General Lee for General Hill, one of his division commanders. These orders exposed the Confederate invasion plan and could have enabled a decisive victory over a divided foe by the Army of the Potomac. However, General McClellan was naturally cautious. He hesitated over how to best take advantage of his captured intelligence, and by hesitating lost the advantage. By the time he engaged Lee's army at Antietam, the confederates were fully concentrated and ready to fight. Instead of the resounding Union victory for which McClellan had hoped, he had to settle for a tactical stalemate.

When the Battle of Antietam was fought, Abraham Lincoln had been considering the possibility of issuing an Emancipation Proclamation, which would free the slaves in the rebellious states, for some time. However, his advisors worried that if the Proclamation were issued following a string of Confederate victories, that it would look like he were acting out of desperation. Following his advisors' council, Lincoln waited for a Union victory. When news of the Battle of Antietam reached Washington, Lincoln recognized his opportunity. While the action in the field had been indecisive, the battle was enough of a strategic victory to give Lincoln the momentum he was looking for. Lincoln put enough spin on the battle to portray it as a Union victory, and several days later issued the Emancipation Proclamation. The Proclamation strengthened morale in a suffering nation and changed the character of the war, making the end of slavery an explicit goal of the United States government. The right opportunity seized at the right moment gave Lincoln considerable momentum moving into the critical year of 1863.

So what? That's what.