In September, 1862, after the second Battle of Manassas, Gen. Robert E. Lee led his victorious Confederates on their first invasion of the North.
At Frederick, Md. he boldly divided his army. Three columns (No. 1) were to surround and capture the Federal garrison at Harpers Ferry and then reunite with Lee and Longstreet (No. 2) at Boonsboro and Hagerstown and continue north.
Gen. George McClellan's strong Union army (No. 3) was hot on Lee's heels, threatening to overrun his divided units. On September 14, Lee was forced to order all available troops around Boonsboro back to defend the passes over South Mountain. Greatly outnumbered, the Confederates desperately held off Union forces until the evening of the 14th, and then withdrew toward Sharpsburg.
On September 15th the Federals poured through the passes, but Lee had gained time to reunite his army.
On the 17th the armies clashed in the bloody Battle of Antietam. The story of the battle is told at Antietam National Battlefield Site, Sharpsburg, Maryland.