Almost in spite of themselves, the carved green ridges overlooking Arlington's Spout Run Parkway are a valued piece of Civil War history.

For two years, Fort C.F. Smith was part of one of the most elaborate defense systems of its time, a 37-mile perimeter protecting Washington. Until the fall of Richmond, Union soldiers manned 19 cannons, maintaining a constant vigil against a Confederate invasion.

Then the soldiers left. Weeds and trees gradually covered the fort. A public street plowed through a third of it. And except for a historic marker, itself framed in foliage, the fort was largely forgotten.

Now, Arlington officials are hoping to restore Fort Smith to its rightful place among local military landmarks, featuring it in a newly purchased public park. County officials and military historians say the fort is one of the few remaining pieces of the original defense line, and the last to be reclaimed from private ownership.

"I think it's an extraordinary acquisition," said Arlington County Board member Albert C. Eisenberg, who has an extensive collection of Civil War memorabilia. "The structure of the fort remains completely clear. In a way, urban development has spread past it, but in some sense time has stood still."

Fort Smith was among the last and most sophisticated of the forts to be built around Washington, said B. Franklin Cooling, chief historian at the Department of Energy. Known as a lunette, it was a roughly C-shaped earthwork, with wooden stockade defenses in the back and an underground haven to protect garrison troops from enemy artillery.

Located on North 24th Street in Arlington's Palisades area above what are now the Spout Run and George Washington parkways, it guarded against attacks on sections of Georgetown and raids through the Spout Run valley, Cooling said.

Civil War historians say Washington's series of surrounding forts, trenches and cannon batteries make up a largely forgotten legacy that literally is woven into the region's landscape.

"There are a lot of rifle trenches that run right through schools and people's yards," said Walton Owen, coauthor of "Mr. Lincoln's Forts," a history of Washington's Civil War defenses. "A lot of people aren't aware these things exist on their own property."

Although Washington fell under direct attack only once during the Civil War, when Confederate Gen. Jubal Early's cavalry attacked Fort Stevens in July 1864, fear of Southern invasion gripped city leaders from the earliest days of the rebellion.

Construction of forts around the perimeter of the city began in earnest after Union troops were defeated at Manassas in July 1861 and continued over the next two years. Eventually, 68 full forts and 93 unnamed batteries ringed the nation's capital.

Although less celebrated than such military installations as Fort Sumter, the local forts played a vital, if quiet, role, Cooling said.

"It was a deterrent," he said. "The combination of forts, cannon and garrisons stymied the Confederates for four years from starting a wholesale attack on Washington."

According to historians and records of the day, life in the fort system was a quiet haven in this country's bloodiest war. So removed from the squalor and violence of battle were the forts that many troops' formal uniforms included white gloves.

Eisenberg owns a letter written by Sgt. Horatio Ginn, a Union volunteer from Maine who thought he was being dispatched to battle, but found he was being assigned to help build forts just across the Potomac from Washington.

"We were supposed to be going into battle, but instead of that they set us to {chopping} trees about a mile from Chain Bridge on a hill," wrote Ginn, who was killed in combat near Fredericksburg in 1863. "We soon had the woods cut down for two or three miles and commenced to build one small fort."

Usually about 10,000 soldiers were involved in defending Washington. Fort Smith was designed to hold 100 soldiers, Cooling said. In 1864, most of the soldiers were reassigned to fight under Gen. Ulysses S. Grant as he fought his way toward Richmond.

With the end of the war, the forts were abandoned altogether, and most simply vanished as the area grew. Only three forts -- Stevens, near Walter Reed Army Medical Center, Foote, near Fort Washington, and Ward, in Alexandria -- are essentially restored to their wartime appearance, Cooling said.

Fort Smith -- named after Charles Ferguson Smith, a general who served as a mentor to Grant and helped him capture Fort Donelson in Kentucky before dying of an infected leg wound -- has survived in "an excellent state of natural preservation," Owen said.

In 1902, there were plans to build an estate on top of it, and county officials later rejected plans to build a housing complex for the elderly. Ultimately, the fort suffered only one great loss to development, when North 24th Street was paved over the southern third of the fort.

Last month, County Board members voted to spend $5.2 million to buy a 14.7-acre farm on which the fort is located. The county takes possession of the land next month, but is likely to keep it closed to the public until a full survey of the site is conducted, said Stan Ernst, a planner for the parks and recreation department.

That it has survived at all is one of the fort's greatest victories, Eisenberg said.

"All history suffers from people's failure to recognize it before it gets lost," he said. "We must recognize how important that conflict was to us, even today."

In the Civil War, Union soldiers at Fort C.F. Smith protected Washington. Now, Arlington officials are hoping to restore the fort to its rightful place among local military landmarks.