SHARPSBURG, MD. -- Just as the opposing armies marched over South Mountain and assembled for the Battle of Antietam 125 years ago, developers are creeping over the western Maryland ridge, threatening the site of the bloodiest day of the Civil War.

''South Mountain has been a natural ally with the battlefield in keeping development out,'' said Dennis Frye, president of the Save Historic Antietam Foundation Inc. ''But it's creeping around the mountain, and that is a dangerous omen. We need to act now -- with a capital N-O-W -- to save Antietam Battlefield, because if we don't act now we will lose it.''

The Battle of Antietam began at dawn on Sept. 17, 1862, on 7,000 hilly acres nestled along the Potomac River about 60 miles northwest of Washington. After the smoke cleared, more than 23,000 Union and Confederate soldiers were killed, wounded or missing.

As the 125th anniversary is commemorated, the land is being threatened by urban development moving from Baltimore and Washington west into the Antietam Valley.

''This is an issue of national ramifications. We are talking about bulldozing our battlefields, and I don't want the responsibility on my shoulders of watching this generation destroy our Civil War history. Americans stained the battlefield there with their blood,'' Frye said. ''There must be a compromise between developers and preservationists. We are not opposed to development, but we do not want Sharpsburg to become another commercial Gettysburg.''

The National Park Service owns about 800 acres outright and controls another 1,200 acres in easements, Frye said. About 2,000 acres of the 7,000-acre historic battlefield is protected.

The lack of development has helped to make it the most well-preserved Civil War battlefield in the East, and perhaps the country.

''The opposite of it is Gettysburg. They have absolutely sold it to the highest bidder,'' said Roger Long, associate editor of Blue & Gray Magazine, based in Columbus, Ohio. ''Everybody's heard of Gettysburg, but not everybody has heard of Antietam, yet it was the single bloodiest day.''

''I have been all alone at Antietam. I've walked up the Bloody Lane, where there were bodies stacked so you could walk and never touch the ground. It's kind of spooky,'' Long said. ''I don't think you could ever do that at Gettysburg no matter what time of the day or what time of the year you were there.''

Besides having South Mountain as a natural barrier to development from the east, farmers who own land surrounding the Antietam Battlefield have not sold their property to developers. They have continued to pass the farms from one generation to another, said Frye.

Development around the battlefield also has been limited because the 800 residents of nearby Sharpsburg have not catered to the thousands of Civil War buffs and tourists who visit Antietam every year.

''In all honesty, we don't see them,'' said Sharpsburg Mayor Gerald Quinn.

''Because we haven't aimed business at them, there is no reason for them to come into town,'' Quinn said. ''If you plan development aimed at bringing people in, you can't pick and choose what comes in. I don't know why anyone would want to go to Gettysburg.''

Gettysburg is about 45 miles north of Antietam. On July 3, 1863, in a last ditch effort to break the center of union lines, Confederate Gen. Robert E. Lee ordered 15,000 men, under Gen. George E. Pickett, to charge the union ranks.

When Pickett's charge failed, the Confederate Army was forced to end its 1863 invasion of the North and withdraw into Virginia.

Today, the northern end of Pickett's charge is heavily developed.

''There's block after block of fast food restaurants, souvenir shops and motels. It has been completely destroyed,'' Frye said. ''Nobody raised their voices in alarm in Gettysburg and, as a result, it is one of our most blanket disasters in terms of historic preservation.''

The Antietam Battlefield, just a little more than an hour's drive from Baltimore and Washington, now could face a similar fate.

Until 1985, proposed development was inside the National Park Service's authorized boundaries, and the park service was able to protect any land proposed for development.

In 1985, however, a shopping center was proposed on the Grove farm outside the park service's boundaries.

The Washington County commissioners rezoned 20 acres of the 120-acre historic tract, just south of Sharpsburg, from agricultural to commercial.

The rezoning alarmed preservationists, who feared the shopping center would start a chain reaction and that the entire strip from Sharpsburg to Shepardstown, W.Va., would fall to commercial development.

The Grove farm was the backdrop of famous photographs taken when President Lincoln traveled to Antietam to confer with Union Commander George B. McClellan.

''We felt the area deserved more than asphalt parking lots, neon signs and souvenir trinket shops,'' Frye said.

Preservationists collected 5,000 signatures in 30 days through Civil War Roundtables across the United States and, after various court battles, the rezoning was nullified. The rezoning issue currently is on appeal.

Meanwhile, the Save Historic Antietam Foundation has continued to fight other zoning battles and its members are eyeing property that is up for sale.

The state recently gave $50,000 to the county, which has set up a special committee to study preservation around the Antietam battlefield and southern Washington County. The committee's first meeting was in late August.

Washington County tourism director Bob O'Connor, who said the battlefield needs some services, is pushing for a formal plan for controlled growth in the area.

Members of the Save Historic Antietam Foundation do not want to see any services unless they can be offered without compromising the historic integrity of Sharpsburg, but they agree that a plan for future development is needed.

''More and more fires are going to come over the horizon,'' O'Connor said. ''I've heard that development up in Frederick County is moving toward us at a rate of 17 feet a day. I don't know where that came from, or where they started marking from, but it's obvious that development is coming this way. The next frontier, so to speak, is here in the valley.''