The Battle of Antietam was fought on September 17, 1862. The United States Army of the Potomac led by General George B. McClellan fought against the Confederate Army of Northern Virginia led by General Robert E. Lee. The battle was fought along the Antietam Creek near Sharpsburg, Maryland. Both of the armies were densely concentrated in the Sharpsburg area, and it was a very bloody battle. The Union Army lost over twelve thousand men, while the Confederate Army lost around ten thousand men. General Robert E. Lee narrowly escaped defeat this battle and the lack of men cause him and his army to retreat back in to Virginia. Lee had good reason for wanting to bring Maryland into the Confederacy. With having Maryland, he would have good location to attack the major cities like Washington D.C. and Philadelphia. It would also give him a chance to get to the rich farmland of the North that would give his army supplies of food. Lee split up his army of fifty thousand men, sending "Stonewall" Jackson to capture the Union arsenal at Harper's Ferry. He told James Longstreet to move north towards Hagerstown, Maryland. Smaller groups were left with the task of guarding against McClellan's troops. Even with all the planning, his adventure seemed to be doomed from the very beginning. The people of Maryland did not give Lee and his Confederate troops a happy welcome. Instead of being treated like heroes as Lee thought, they were treated like invaders. Even the secessionist from Maryland did not like the idea of the Confederacy invading their state. Lee was setback once again when a letter containing his plan of attacks and the locations of all Confederate troops were found by a Union private near Frederick, Maryland. If McClellan had moved quickly, he could have easily crushed Lee's army and ended the whole war altogether. But McClellan did not move quickly enough and within twenty-four hours, Lee learned of his danger and pulled his troops to Sharpsburg.