Since Antietam, is literally across the river from Shepherdstown (Southern dead are buried here) the pronunciation has always been Ann TEE Tum.

A couple of tid-bits, Little Hawk:

Lee's Orders were found wrapped around a couple of cigars, by a couple of Union skirmishers, near the battle of South Mountain, a preliminary fight. Longstreet had captured Harpers Ferry to protect Lee's right flank. (Harpers Ferry changed hands some six times during the War).

Several Union failures allowed Lee to get away:

Burnside failing to cross the Antietam Creek, at what is now Burnside's Bridge, until late in the afternoon. He had several divisions which were held off by some 300 Texan sharpshooters, and could have crossed some 400 yards up stream but continued to try and cross the bridge.

McLellen commiting his forces piecemeal. First in the north at the Cornfield, next in the middle at the Dunker Church, then finally in the south at Burnside's Bridge. Lee hit them in the north, next swung his troops to the middle, stopped them at the Church, but lost ground at Bloody Lane. Finally in the afternoon, Burnside crossed in the south and was prepared to deal the exhaused Southerners a fatal blow, when Longstreets divison (thereafter called the Foot Cavalry)from Harpers Ferry, slammed into the Union troops and drove them back two miles. Lee crossed back into the South at Shepherdstown, and when McLellen started a pursuit, the battle of Shepherdstown took place, where the Southerners drove the Union back a second time.

A final tidbit. The Antietam Battlefield Cemetary contains only Union dead, and one recent burial. When Patrick Roy was killed on the USS Cole, his father lived in Shepherdstown and his mother lived in Keedysville, within sight of the Cemetary. The area congressmen got together and had one grave site opened. Patrick had worked at O'Hurleys General Store before going into the Navy, and had many friends in town. His grave sits next to a walled overlook, so his mother could see it from her house (she has since moved away). At the grave site his brother screamed out "Kill every raghead with a gun". As the local paper recently reported, returning West Virginia National Guardsmen have been leaving scraps of rags on his grave...