The 20th Maine Infantry Regiment was a volunteer regiment of the United States Army (Union Army) during the American Civil War (1861–1865), most famous for its defense of Little Round Top at the Battle of Gettysburg in Gettysburg, Pennsylvania, July 1–3, 1863. The 133rd Engineer Battalion of the Maine Army National Guard and the United States Army today carries on the lineage and traditions of the 20th Maine. 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 that time, the brigade also 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 in July 1863, the regiment was held in reserve at Antietam in September 1862. The regiment was also among those forced to remain overnight within sight of the Confederate lines at Fredericksburg in December 1862, 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 in April-May 1863, 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 and commander of the regiment by Lt. Col. Joshua L. 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 at Gettysburg, Pennsylvania, where it was stationed on Little Round Top hill 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 up the slope at them by instead suddenly charging downhill with fixed bayonets, surprising and scattering the Confederates, thus