The 20th Maine Volunteer Infantry Regiment was recruited in Piscataquis, Penobscot, Lincoln and Knox Counties in Maine in August 1862 in response to President Lincoln's July 1862 call for 300,000 troops. The unit, consisting of 1,621 farmers, shop clerks, woodsman and seaman and commanded by Lt. Colonel and May 6, 1861 West Point graduate Adelbert Ames, was mustered into federal service August 29, 1862. Bowdoin College professor Joshua Lawrence Chamberlain, served as the regiments colonel under Ames.
The 20th Maine joined the 1st Brigade, 1st Division V Army Corps, commanded by Major General Fitz John Porter prior to the Battle of Antietam, where they were held in reserve. The regiment saw action at Fredericksburg but was relegated to guard duty at Chancellorsville because many of the soldiers had come down with smallpox.
In May 1863 Adelbert Ames was promoted to Brigadier General and the colonelency of the 20th Maine devolved upon Joshua Chamberlain. Chamberlain would lead the regiment in to Battle at Gettysburg as part of Colonel Strong Vincent's 3rd Brigade, 1st Division, V Corps.
The Battle of Gettysburg opened on July 1, 1862 while the 20th Maine was still en route to the battlefield. They arrived early in the morning of July 2, after an arduous 25 mile march and before noon rested in a field west of the Baltimore Pike behind the union lines on Cemetery Ridge.
The Union Army held a strong defensive position on July 2nd but problems arose when Major General Daniel Sickles did not align with the right of the II Corps which formed a line along Cemetery Ridge. Instead he moved his troops westward into a position that left both his flanks exposed a move which in turn threatened the left flank of the Union Army. A second threat to the Union Army left flank was the fact that the high ground on Little and Big Round Top was not defended and was being threatened in the early afternoon by Confederate troops under James Longstreet. Brigadier General Gouverneur K Warren realized this and because Sickles could not provide support he obtained troops from the V Corp who were ordered to ascend Little Round Top and protect the army's left flank. The 20th Maine was one of the regiments dispatched to Little Round Top with the order to hold the extreme left flank of the army at all hazard.
The 20th Maine was positioned on the south end of Little Round Top by Colonel Chamberlain with their left flank resting on the east side of the mountain and their right on the west side. The regiment withstood a handful of assaults by the 15th Alabama and with a third of their number dead or wounded and ammunition running low Chamberlain ordered a bayonet charge which finally drove back the Alabamians and saved the Union left flank. Chamberlain would later become famous for his exploits at Gettysburg, amongst other things, as would the 20th Maine to a lesser extent.
Today there are markers on Little Round Top that show the position of the 20th Maine's right and left flank and a monument dedicated to the regiment within their lines. Remnants of the rifle pits they manned on that hot July day in 1862 still remain on the summit.