20th Maine Volunteer Infantry Regiment The 20th Maine Volunteer Infantry Regiment was a regiment of the United States Army during the American Civil War , most famous for its defense of Little Round Top at the Battle of Gettysburg , July 1863 . Organization . The 20th Maine was organized in the state of Maine and mustered into Federal service on August 29 , 1862 , with Col . Adelbert Ames as its commander . It was assigned to the Army of the Potomac in the 3rd Brigade , 1st Division , V Corps , where it would remain until mustered out on July 16 , 1865 . At the time the Twentieth joined the brigade it consisted of the 16th Michigan , the 12th , 17th , and 44th New York , 83rd Pennsylvania Volunteer Infantry regiments , and a Michigan company of sharpshooters . Combat history . Prior to their notable actions at Gettysburg , the regiment was held in reserve at Antietam , was among those forced to remain overnight within sight of the Confederate lines at Fredericksburg , forcing the regiment ' s Lt . Col . Joshua Chamberlain to shield himself with a dead man . The unit was unable to participate in the battle of Chancellorsville due to a quarantine prompted by a tainted smallpox vaccine that had been issued to the unit ' s soldiers . On May 20 , 1863 , Colonel Ames was promoted and was succeeded as colonel of the regiment by Joshua Chamberlain , who had been offered and declined leadership of the unit at the time it was formed . Gettysburg and Little Round Top . The most notable battle was the regiment ' s decisive role on July 2 , 1863 , in the Battle of Gettysburg , where it was stationed on Little Round Top at the extreme left of the Union line . When the regiment came under heavy attack from the Confederate 15th and 47th Alabama regiments ( part of the division led by Maj . Gen . John Bell Hood ) , the 20th Maine ran low on ammunition after one and a half hours of continuous fighting ; it responded to the sight of rebel infantry forming again for yet another push at them by charging downhill with fixed bayonet s , surprising and scattering the Confederates , thus ending the attack on the hill . The 20th Maine and the adjacent 83rd Pennsylvania together captured many men from the both Alabama regiments ( including Lt . Col . Michael Bulger , commander of the 47th ) , as well as several other men of the 4th Alabama and 4th and 5th Texas regiments of the same division . Had the 20th Maine retreated from the hill , the entire Union line would have been flanked , and would have most likely lost the battle of Gettysburg , although there is much debate on the subject . Union morale and support for the war was wavering , due to many defeats . If the Union had lost the battle of Gettysburg the Confederate army could have possibly marched on to Washington D . C . and ended the war . The 20th Maine ’ s action in holding the hill has been credited with helping to turn the tide of the war . It has been suggested that Lt . Holman S . Melcher initiated the charge , although Col . Chamberlain has been credited by most historians for ordering the desperate attack . This action is depicted in the novels " The Killer Angels " and " Courage on Little Round Top " , and was subsequently an important scene in the movie " Gettysburg " . Later War . Later actions in which the regiment participated included Rappahannock Station , Mine Run , the Wilderness , Spotsylvania , North Anna , Totopotomoy , Cold Harbor , Petersburg , Five Forks , and Appomattox . Disbandment . The 20th Maine marched from Appomattox , Virginia , on May 2 , reaching Washington , D . C . , on May 12 , where the remaining original members were mustered out of service on June 4 , 1865 , with the remainder of the regiment leaving the service on July 16 . The 20th Maine had a total wartime enrollment of 1 , 621 men ( including initial muster , replacements , and drafts ) , losing 147 dead from combat , 146 dead from disease , 381 wounded , and 15 in Confederate prisons .