September 17, 1862
The Battle of Antietam (also known as Sharpsburg) was the 20th Maine's introduction to the field of battle. Although they took no part in the action on what would turn out to be the bloodiest day in American history, they were present on the field. The Fifth Corps was held in reserve by McClellan, but many of the 20th's officers, Chamberlain included, watched the battle unfold from a rise overlooking the Stone Bridge.
On September 20th the regiment took part in a reconnaissance mission across Antietam Creek. It ended in a retreat back across the stream, but was memorable for one event. That day Lt. Colonel Chamberlain was to have the first of what would be several horses shot out from under him.
National Park Service - Antietam National Battlefield
Source: Alice Rains Trulock; In the Hands Of Providence