Antietam

Being an old Civil War buff, I drove up to the Antietam battlefield on Monday. It's in Maryland, about an hour's drive from DC, and very close to the Virginia border. Lee was invading the North (Maryland remained in the Union despite being south of the Mason Dixon line). Lee was north and west of both Washington and Baltimore, and threaten to capture either or both cities. Taking Baltimore would have been nearly as good as taking Washington, the only route to Washington ran thru Baltimore.
The land out in western Maryland is low and rolling, a lot of open fields, cut by little streams, sunken roads, and woodlots. From Union headquarters you cannot see the entire battle field. McClellan must have relied upon messengers to learn where his units were and where the enemy was. Messengers are slow, apt to get lost or shot, or forget to pass on vital information. Battlefield command could be difficult back then.
Terrain features played decisive roles. I saw the Bloody lane where the Confederates mowed down the Union infantry. It's just a one lane wagon track, sunk about a man's height below the fields. The banks slope up at about 30 degrees. Standing in the road, the confederates were protected from rifle fire, and could load and fire and do it again. There is a contemporary photograph showing the field in front of the Bloody Lane covered with Union dead.
Then there is Burnside's Bridge over the Antietam creek. Burnside's entire corps of 9000 men took from 9 AM til 1 PM to force their way across this bridge against 2500 Confederate defenders. Burnside was something of a chucklehead. He never realized that Antietam Creek is shallow enough to wade across. Burnside could have waded across on a broad front and overwhelmed the confederates in less than an hour. Concentrating on crossing the bridge held Burnside up and cost terrible losses.
Antietam was a very important battle. It came after a year of defeats at the hands of Lee and Jackson. Lee felt strong enough to invade the Union rather than just standing on the defensive in Virginia. But despite a bloody year of losses, the Union was able the throw a strong army right into Lee's path, and make frontal attacks on Lee's lines. Lee was driven back to Virginia only a few miles from the Virginia border.
Antietam was the victory upon which Lincoln hung the Emancipation Proclamation. He had been ready to issue it earlier that year, but his cabinet pointed out that proclaiming freedom for slaves after a summer of military defeats would look like a desperation move on the part of the Union. It would be seen as an attempt to raise a slave insurrection in the South after conventional warfare had failed. As it was, after a solid (if costly) Union victory, the Emancipation Proclamation announced the adherence of the Union to the principle of the Declaration of Independence ("All men are created equal").