The Battle of Antietam was the bloodiest one-day battle in the history of American wars. It was fought on September 17, 1862. The battle was not really a tactical victory for either side, but it ended up as a strategic victory for the North.
As with the later Battle of Gettysburg, the Battle of Antietam came about because of a Confederate invasion of the North (the battle was fought in Maryland, a Union state). The Confederates hoped at the very least to gain support from France or Great Britain if they could successfully invade the North. They hoped the Europeans would decide that the South could win the war. If the Europeans did so, and if they recognized Confederate independence, the North would be pressured to end the war.
As Lee invaded, his plans were actually dropped by a courier and found by the Union. This allowed Union General George McClellan to know what Lee planned. Even so, McClellan was too hesitant and was therefore not able to really destroy Lee’s army. Instead, in the one day of fighting, the two sides sustained similar numbers of casualties.
The battle is seen as an important turning point in the war. It stopped Lee’s invasion. It also encouraged people in the North and improved public support for the war. It inspired President Lincoln to fire McClellan. Finally, it allowed Lincoln to claim a victory which enabled him to issue the Emancipation Proclamation (he did not want to issue it when the Union seemed to be losing because he didn’t want to look desperate). This changed the meaning of the war completely.