An-tee'-tum, as Nancy King posted, with the second "t' only slightly pronounced. The name is from well outside of the Dutch area; probably a corruption of an Indian name. The battlefield at Antietam (Sharpsburg) is being restored with original fence lines, woodlots, etc., the Park comprising almost 3300 acres.
Sharpsburg is a town on the edge of the battlefield, hence the alternate (and first) name for the battle.