The fighting that killed or wounded 21,000 Americans in the rolling hills of western Maryland was over in about 12 grisly hours.

But a century and a half after the bloodiest day in American military history, the struggle to preserve the ground where Union and Confederate soldiers fought the Battle of Antietam only now appears close to a declaration of victory.

As Americans gather to honor the sacrifice of those who fell Sept. 17, 1862 — as they are doing this weekend and Monday on the 150th anniversary — they will do so at one of the nation's best-preserved Civil War sites....