The selection was based on the park's request for money to purchase land, said spokeswoman Susan Hawley.
Howard said the National Park Service wants to protect the remaining 800 acres it doesn't own within the 3,200-acre park, but there's no immediate development threat.
"There was no fire. I hate when people do this because they're not really aware of the long-term plan," he said.
The park has $1 million that it's using to slowly and steadily buy land from owners who are willing to sell over the next two years, he said.
State and local preservation groups have protected from development more than 4,300 acres surrounding the park.
Local historian Dennis Frye said the group was way off base in singling out Antietam.
"This kind of false information should not be placed in public. This results in unnecessary fear tactics. If anything, Antietam should be heralded as a champion site," Frye said.
Frye questioned why Harpers Ferry National Historical Park wasn't on the list.
Antietam has done such a good job of preserving the land in and around the battlefield that the board of directors of the Civil War Preservation Trust is visiting the park on Sept. 17, the 138th anniversary of the battle, Howard said.
There was a time when Antietam was in danger.
From 1988 to 1991, the National Trust for Historic Preservation put Antietam on its list of most endangered sites. That was a listing the park took seriously.
But the group now calls Antietam a success story because of preservation efforts led by the state of Maryland and the Save Historic Antietam Foundation.
"Antietam is, of all the national parks, probably one of the best in terms of preservation. I think Washington County should be proud of that," said President Tom Clemens.
The Chesapeake and Ohio Canal National Historical Park, with headquarters in Sharpsburg, was also on the National Park Trust's priority list.
Park Superintendent Doug Faris said there is no immediate development threat to the 19,000-acre park.