GO OVER the ground at Gettysburg at dawn or dusk and you're likely to encounter only browsing deer and, perhaps, the ghosts that guard the fields where force of arms decided, once and forever, that these united states are one nation, indivisible.

Although more than a million visitors tour the battlefield every year, nearly all come and go by daylight. To do so is to miss the essence of that hallowed ground, best sensed in the uncertain light at the beginning or end of the day, when modern details are muted and the eye and the mind are free to people the shadows with soldiers.

Gettysburg draws more visitors than any other Civil War battlefield because what happened there is both clear-cut and incredible. Even those who know very little about the war have heard of Pickett's charge and understand that the Cnfederate dream melted away with those suicidally stubborn Southerners that awful afternoon. Yet the more one learns about the events of those first three days of July 1863, the less possible it seems that so many men could have been so brave, and so many of their leaders so foolish.

The course of the three-day battle is fairly easy to follow. Lee, marching blind without his cavalry, was fattening his fine but ragtag army on Pennsylvania's milk and honey, and hoping to frighten the North into a negotiated peace. He thought the Union army was still down in Virginia, and only learned that the Yankees were hot on his trail when they trod on his tail at a little crossroads town nobody had ever heard of.

On July 1, having "got thar fustest with the mostest," the Southerners drove the leading units of the Federal army reeling through Gettysburg, then failed to make the one last push that might have dislodged the Yanks from a ridgetop refuge and routed the whole Union army, opening the way to Washington and the last, best hope for Southern independence.

On July 2, Lee attacked the ends of the Federal line. Both of the assaults failed, but it was a near-run thing: the Confederates came within a few minutes of turning Meade's misplaced left flank and within a few yards of turning his right.

On July 3 came Pickett's charge, which in an hour cost 8,000 casualties among 12,000 men (some say 10,000 of 15,000). It probably would have failed even if it had succeeded, because there were fresh Federal troops in position to contain and crush any breakthrough the exhausted Southerners could conceivably have made.

It was over. During those three days, 51,000 men had been killed, wounded or captured, Lee losing one man in three and Meade one man in four. The Union's legendary Iron Brigade had suffered 80 percent casualties in a couple of hours; the 1st Minnesota lost as many in a few minutes as it rushed to fill a gap in the line. The casualty rate of the 14th Tennessee was over 99 percent: three men left of 365.

At this distance in time it's hardly possible to understand how the soldiers could keep on keeping on as the dead and dying accumulated in windrows, but it is dreadfully fascinating to wander over the fields and ridges where they fought.

First, though, stop at the visitor center for the electric-map lecture, and at the Cyclorama center to see Paul Philippoteaux's vast painting of the climactic moments of Pickett's charge. Most of the town's commercial museums and exhibition theaters are also worth visiting, and there are package plans that cut admissions by a third.

Then take a bus tour, or better yet go in your own car with a federally licensed guide from the National Park Visitor Center (717/334-1124, ext. 31). This personal tour costs $12 for up to five persons, lasts a couple of hours and is likely to yield wonderful sidelights and details. Most of the guides have spent years studying the men and movements of the battle; some can, if you like, recount almost step-by-step the part played by virtually any unit on the field. And you can set your own pace, concentrating on what most interests you.

Once the course of the action is clearly in mind, go alone, on foot or bicycle, with map and flashlight. Start over the field of the First Day (northwest of the town) in late afternoon, following the well-marked and heavily monumented tourway.

Coming through town, note the buildings bearing brass plaques certifying they predate the battle; many are marked by bullets and shrapnel, and some have cannon shells imbedded in their brickwork. It still seems fantastic that only two civilians were killed as the battle surged through the town, which became a charnel house. By July 4, virtually every church, store, home, office, shack and shed was full of wounded men, 26,000 of them, more than ten times the normal population of the town. Dripping wagons rattled through the streets day and night, bearing away dead men and amputated arms and legs.

Come nightfall you'll be at Cemetery Hill, tired and hungry and ready to pack it in (as General Ewell did at nightfall of the First Day, then and there losing the battle).

Be on Little Round Top, the southern end of the Union line, before dawn (the park opens at 6). Sit quietly with your back against one of the boulders just below the brow of the hill, where the Union defenders were posted, and listen. Wild turkeys may call sleepily from a roost on Big Round Top, and as the light comes up, try to see the deer browsing in the fields below. They will be there -- there are roughly five hundred of them in the park -- but they're virtually invisible, even through binoculars, because their gray-brown coats blend into almost any background. Most Confederate uniforms were about the same color.

Leaving Little Round Top, take a few minutes to follow the path to the monument that stands where the Twentieth Maine Volunteer Infantry was posted on the Union extreme left flank. Led by a professor of rhetoric from Bowdoin College, the regiment held the fate of the army in its hands on the afternoon of the Second Day; after half the men had fallen and the ammunition had run out, the rest fixed bayonets and charged, shattering the final Southern attack on that flank.

Sheep now safely graze in the Valley of Death below Little Round Top, gleaning grass among the boulders along Plum Run, a miserable creek the soldiers called Bloody Run. In the rains that followed the battle, it carried the life's blood of thousands of them away to the Potomac.

Except on the modern roads, there's no decent footing anywhere around Devil's Den, the Peach Orchard and the Wheatfield, yet again and again on the Second Day some men ran to the attack, some ran away and others manhandled heavy cannons over ground where strong men could hardly go. It is necessary to cross that ground on your own two feet to appreciate what those arrows on the battle maps signify. Dotting the woods and fields are more than 2,000 monuments, ranging from flank markers to elaborate altars to the memory of men and units that were engaged on this spot on that day. Descendants of some of those soldiers still polish the brass plaques bearing their names.

By now it's broad daylight. Follow the Wheatfield Road to West Confederate Avenue and the Virginia memorial, atop which sits the mounted Lee, forever gazing over the ground where, on the Third Day, he ordered the final, crazy assault, over the desperate objections of Longstreet, his best and most trusted general. What raises the slaughter to tragedy is that Lee ordered the attack in spite of his love for his soldiers, and they obeyed him because of their love for him.

"It's all my fault," the distraught Marse Robert said again and again as he comforted the survivors who came stumbling back through the smoke. That night the Confederate wounded who lay within Lee's lines were started toward Virginia. The wagon train was 17 miles long, and ook 26 hours to pass a given point. They had to leave their dead for the Yankees; among the thousands of bloating corpses that marked the path of Pickett's charge was that of at least one female Southern soldier, her secret revealed by the yielding of her uniform.

At Trinity Church in Gettysburg, the operating tables were doors laid across the backs of the pews. Holes were bored in the floor to drain away the blood.

READY READING

The following books, available in paperback in the bookstore of the National Park Service visitor center at Gettysburg, are among the many excellent sources on the battle:

GETTYSBURG: THE FINAL FURY by Bruce Catton (Berkley, $6.95). A brief, clear and moving account by the best-known Civil War historian.

GETTYSBURG: A JOURNEY IN TIME by William A. Frassanito (Scribner, $14.95). The definitive book of Gettysburg battlefield photographs.

THE TWENTIETH MAINE by John J. Pullen (Morningside, $10.95). A fine account of a famous regiment that fought from Antietam to Appomattox, full of fascinating detail and insight into the citizen soldier.

THE KILLER ANGELS by Michael Shaara (Ballantine, $3.50). A wonderfully written novel about the Gettysburg campaign that won a Pulitzer Prize and has been praised as "more true than history."