The Battle of Antietam (called the Battle of Sharpsburg in the South; note that the Union tended to name battles after area natural features while the Confederacy tended to name battles after nearby towns) followed several Confederate victories, and although it was basically a tactical draw, the Union government made good use of it politically (prior to Antietam, some European countries were considering recognizing the Confederate States of America; after Antietam, they dropped all such considerations). The Battle of Antietam lasted just one day, September 17, 1862. It was the bloodiest day in the history of American warfare. While Gettysburg posted the most casualties of any American battle, it lasted a full three days. In the Battle of Antietam, there were around 23,000 casualties in just one day. The Confederate leader was General Robert E. Lee, and the Union leader was General George McClellan.
It took place near the town of Sharpsburg near the Antietam Creek in Maryland. The local inhabitants were pacifists known as the "Dunkers". Their little white church (pictured) was central to much of the fighting.
Union forces had accidentally stumbled upon a copy of the Confederate plans and thus had massed troops in the Antietam area. Gen. Lee had split his army into two groups, one near Antietam and one in Harpers Ferry. The Union plan was to engage the Antietam group before it could regroup with the rest of the army. Things did not work out exactly as planned; partly because Gen. McClellan grossly overestimated the size of the Confederate Army and acted too cautiously and too slowly. (At this time the Federal Army had around 87,000 men while the Confederate Army had approximately 40,000 men.)
Some of the bloodiest fighting took place in a corn field. The species of corn being grown stood around eight feet tall, and at first served as good cover for soldiers. Throughout the day the field changed hands over a dozen times, and in the end the corn had been stripped down by the bullets and the field was strewn with bodies. Another area visciously contested was the so-called sunken road, a short street sunken below the land around it from erosion. Before the day was out, it would be nearly filled with the dead. The bridge pictured is today called the "Burnside Bridge" after the Union General Ambrose Burnside who took it after hours of fighting to cross Antietam Creek and provide Union reinforcements. During that entire time, the bridge was held by only around five hundred or so well-placed sharpshooters on the other side.
Many volunteers from Massachusetts fought in Antietam and there are some monuments so dedicated (there are over 300 monuments in all at Antietam).