In a raid on July 11, 1864, Confederate General Jubal Early attacked Fort Stevens, the northernmost of the defensive ramparts ringing DC. A small but fierce battle raged for two days – the only time the Civil War touched District soil – until Early’s men withdrew across the Potomac. The fort has been partially restored, but the site is pretty neglected overall.
Abraham Lincoln himself was drawn into the shooting: the president, observing the battle from Fort Stevens’ parapet, popped his head up and almost got hit by sharpshooters. It's the only time in American history that a sitting president came under direct fire from an enemy combatant.