What are facts about the Battle of Antietam?

This battle was, of course, a battle in the Civil War.  It was fought in September of 1862.  The battle was fought in the western part of the state of Maryland.

During this part of the Civil War, the South was on the offensive.  Robert E. Lee, in charge of the Army of Northern Virginia, invaded the North (Maryland was a Union state).  By defeating Lee and his army at Antietam, the Union, led by George McClellan, was able to stop this first attempt to invade its territory.

This was a horribly bloody battle, with over 23,000 out of the 100,000 men involved getting killed or wounded.