Between Boonsboro and the Potomac River, which forms Maryland’s border with West Virginia, Antietam National Battlefield preserves the hallowed ground where over 23,000 men were killed or wounded on the bloodiest single day of the Civil War—September 17, 1862. Atop a shallow hill at the middle of the park, a scant mile north of Sharpsburg off Hwy-65, the visitors center (301/432-5124, daily, $10 per car) offers a film, museum exhibits, and interpretive programs that put the battle into military and political context. Though there was no clear winner, Antietam is said to have convinced Lincoln to issue the Emancipation Proclamation, officially freeing enslaved people in Confederate states and effectively putting an end to British support for the southern side.