SOUTH MOUNTAIN, MD. -- One hundred twenty-five years ago, a small band of rebels held off the march of a larger Union force here, giving Confederate Gen. Robert E. Lee time to regroup before the two armies clashed at Antietam Creek on Sept. 17, 1862, in the bloodiest single-day battle of the Civil War.

Today, metropolitan Washington is marching relentlessly westward on this rustic area about 60 miles northwest of the District. Nearby Middletown Valley has recently yielded ground to development, and a second battle of South Mountain is now being fought over its official designation by the National Park Service as a national historic landmark.

Supporters see such action as a way to discourage the development that has overwhelmed historic sites closer to large cities. But property owners here on the border of Frederick and Washington counties think such status will only attract more tourists to their still-quiet mountain and may be a prelude to federal acquisition of their land.

The battle is being fought on both sides of the mountain. East of the three strategic gaps that figured prominently in the Civil War battle, the town of Myersville recently voted to double its size, while residents of Middletown to the south, almost engulfed by new subdivisions, recently voted against allowing more growth.

And west of the mountain, a landowner wants to sell three-acre building sites at the rustic setting of Crampton's Gap, just beyond Bill and Mary Van Gilder's Toll House Gallery and Pottery place. The pristine panorama of the countryside surrounding the Antietam battlefield itself is also threatened by plans for a towering television antenna and for a small shopping center at a farm where President Lincoln met Gen. George McClellan after the battle.

"It's absolutely frightening that this kind of just steady, uncontrolled march of metropolitan Washington now reaches the Blue Ridge and is ready to leap over it," said Jerry Rogers, Park Service associate director for cultural resources. "We're extremely concerned about our Civil War-related sites."

Last week, despite opposition from property owners, a National Park Service advisory committee, meeting at Death Valley, Calif., recommended that 2,000 acres of South Mountain be declared eligible for historic landmark status. The action means that any federally supported project in the area -- roads, for example -- could not go forward without regard to its impact on the historic area.

The committee made its recommendation to the secretary of the Interior, who routinely concurs. Park Service officials had hoped for an outright designation of the site as a historic landmark, but the panel was blocked from taking such a step by the opposition of a majority of property owners -- in this case, 32 of the 47 affected parties. Landmark status does not automatically preclude development, but preservationists said it would give added weight to their case when dealing with local zoning officials.

The difference between the two kinds of historic designation is "a very fine point in law," according to Park Service historian Jim Charlton. But that distinction has been lost on the opponents.

Meanwhile, the South Mountain dispute has raged in the letters column of local newspapers and focused public attention on the creeping suburbanization on Washington's western frontier. It has also highlighted the vulnerability of long-ignored but prime Civil War sites to development.

"Our best protection is a citizen who feels {his property is} historic and will protect it," Rogers said. "We can't buy it all."

In fact, Rogers said, the Park Service wants to buy none of it, but such assurances have done little to assuage the suspicions of property owners who believe that official recognition as a historic site is the first step toward confiscation.

"Some are branding us troublemakers," said Richard Robertson, president of the South Mountain Citizens Association, formed as a result of the dispute and opposed to any landmark status. "We're just trying to protect our rights."

In defense of those rights, Robertson, a home-improvement contractor who moved here from Montgomery County, has a large signboard by his property with three "No Trespassing Signs," one "Beware of the Dog" sign and a fifth sign that says, "Forget the Dog. Beware of the Owner."

Robertson said the issue came home to him in the wake of recent battle reenactments. One night, he said, a Civil War buff marched into his house, unannounced: "He said, 'I thought this was an antique store.' "

Any designation of the area as historic, Robertson said, would inevitably bring tour buses and carloads of history buffs along the narrow mountain roads. The next thing, he said, the government would want land for plaques and parking.

"We should really be pulling together," said George Brigham Jr., whose home was a hospital during the war. "There are going to be people out there regardless. We have to make it easier for them to identify the {historic} sites without disturbing the private property owners."

The commuter traffic on Alt. Rte. 40 heading toward Washington is already getting much worse, he said. "It starts at 4:30 in the morning and lasts until after 8 a.m."

But Robertson and other landmark opponents, seeing tourists as a greater threat, collected more than 300 signatures against any government action.

"The whole tone of the discussion degenerated" because opponents misunderstood the meaning of the historic designation, said Daniel L. Rosvold, another Montgomery County emigrant and property owner who supported landmark status. "People are searching for conspiracies at every level."

Rosvold, who moved to the mountain in 1975, said landmark status could indirectly slow development because of its "effect on the minds of county zoners and planners" who would think twice about it if the question arose.

"I get people coming to my place now and then for relics," Rosvold said. "They're always courteous to me. I don't have any problem with someone coming up to see where his great-granddaddy died. There is a right to be remembered if you die on a battlefield, and there is a right to remember."

Proponents of historic status point to Manassas battlefield in Northern Virginia as a positive example of government protecting historic sites from development, and they cite the construction of town houses and a road through another Civil War site in suburban Chantilly as a bitter fruit of laissez faire.

Opponents invoked the Cuyahoga River Valley near Cleveland where creation of a national recreational area displaced 325 property owners.

On Nov. 5, the Frederick County commissioners unanimously sided with the preservationists, voting to support landmark status for South Mountain.

At the Van Gilders', the potters continue to collect signatures to preserve the battlefield. So far, more than 200 people, many from out of state, have signed. Had the battle, which resulted in 4,500 casualties, ended differently, the petition notes, the war might have ended sooner and Antietam, where 23,000 troops fell, would not have occurred.

"Today, the battlefield lies unprotected from the ravages of development, profiteering and insensitivity," the petition states.

Just beyond the shop, Robert Millard plans to sell 10 home sites soon. "I don't see why we should put artificial restraints on change," he said. "Who thought Washington would build up? This area is bound to change."

But the preservationists aren't ready to surrender.

"I consider eligibility {for historic status} a victory," said Dennis Frye, whose family settled in the area more than 200 years ago. Pledged Frye, a Park Service employe who nominated the mountain for landmark status, "The battle will rage on and on . . . We are continually one step ahead of the bulldozers."