The 20th Maine was organized in the state of Maine in 1862. It became part of the 1st Division of the V Corps of the Army of the Potomac. The regiment served at Antietam, Fredericksburg, Chancellorsville, Gettysburg, Rappahannock Station, Mine Run, the Wilderness, Spotsylvania, North Anna, Totopotomoy and Bethesda Church, Cold Harbor, Petersburg, Five Forks, and Appomattox.
At the Battle of Gettysburg, the regiment under the command of Colonel Joshua Lawrence Chamberlain, was stationed on Little Round Top at the extreme left of the Union line. It was here that the regiment fought its most famous action. When the regiment came under heavy attack from the Confederate 15th Alabama, which was attempting to flank the Union position, and completely ran out of ammunition, it responded by charging downhill with fixed bayonets, empty rifles, and in desperate hand-to-hand combat ending the Confederate attack and turned the tide of the battle. This famous action is depicted in the novels “The Killer Angels,” and “Courage on Little Round Top”, and subsequently became an important scene in the movie “Gettysburg.”