Satisfied that he had taken McClellan's measure, Lee slipped away from Richmond and struck the decisive blow at SECOND MANASSAS, then led his tired, ragged and hungry army on to invade the north. He circled to the west of Washington, taking HARPERS FERRY en passant, and crossed the Potomac near Sharpsburg, Maryland. Confident it would take the Union army months to get its act back together, Lee violated every rule he had learned -- and taught -- at West Point by dividing his army into five parts while in hostile territory (although it should be granted that he expected the border-state Marylanders to welcome him). McClellan reorganized the Union army almost overnight and raced to intercept Lee, but then threw away his chance to destroy the scattered Confederates and, very likely, to kill or capture Lee and most of his generals in the bargain. The Battle of Antietam -- had it begun on September 15, when the Federals had Lee and his relative handful of troops pinned against the Potomac -- might have ended the war, and certainly would have shortened it. But McCLellan dawdled for two days and then attacked piecemeal. By sundown on September 17, more than 23,000 men had died in the deadliest day of the war. McClellan still had ample uncommitted reserves, but lay on his arms all the next day. Finally, Lee withdrew into Virginia. Most of the battlefield remains largely unchanged, although much of it is in private hands. The NPS staff there is particularly knowledgeable and helpful. ANTIETAM NATIONAL BATTLEFIELD -- About 11/2 hours (60 miles) from Washington via I-270, U.S. 40 west and Route 34 south.