Chamberlain believed the Union needed to be supported by all those willing against the Confederacy. On several occasions Chamberlain spoke freely of his beliefs during his class urging students to follow their hearts in regards to the war while issuing his own proclamation that the cause was just. Of his desire to serve in the War he wrote to the Governor of Maine, "I fear, this war, so costly of blood and treasure, will not cease until men of the North are willing to leave good positions, and sacrifice the dearest personal interests, to rescue our country from desolation, and defend the national existence against treachery." Many faculty at Bowdoin did not feel his enthusiasm for various reasons and Chamberlain was subsequently granted a leave of absence (supposedly to study languages for two years in Europe). He then promptly enlisted unbeknownst to those at Bowdoin and his family. Offered the colonelcy of the 20th Maine Regiment, he declined, according to his biographer, John J. Pullen, preferring to "start a little lower and learn the business first." He was appointed lieutenant colonel of the regiment on August 8, 1862, under the command of Col. Adelbert Ames. The 20th was part of the V Corps in the Union Army of the Potomac. One of Chamberlain's younger brothers, Thomas Chamberlain, was also an officer of the 20th Maine, and another, John Chamberlain, visited the regiment at Gettysburg as a member of the U.S. Christian Commission until appointed as a chaplain in another Maine Volunteer regiment.
Chamberlain's regiment marched to the Battle of Antietam, but did not participate in the fighting. The 20th Maine fought at the subsequent Battle of Fredericksburg, suffering relatively small numbers of casualties in the assaults on Marye's Heights, but were forced to spend a miserable night on the freezing battlefield among the many wounded from other regiments. Chamberlain chronicled this night well in his diary and went to great length discussing his having to use bodies of the fallen for shelter and a pillow while listening to the bullets zip into the corpses. The 20th missed the Battle of Chancellorsville in May of 1863 due to an outbreak of smallpox in their ranks (which was caused by an errant smallpox vaccine), which kept them on guard duty in the rear. Chamberlain was promoted to colonel of the regiment in June 1863, upon the promotion of Ames.
Chamberlain achieved fame at the Battle of Gettysburg, where his valiant defense of a hill named Little Round Top became the focus of many publications and stories, including the novel The Killer Angels and the film Gettysburg.
Chamberlain's position on Little Round Top
On the second day of the Battle of Gettysburg, Union forces were recovering from initial defeats and hastily regrouping into defensive positions on a line of hills south of the town. Sensing the momentary vulnerability of the Union forces, the Confederates began an attack against the Union left flank. Sent to defend the southern slope of Little Round Top by Col. Strong Vincent, Chamberlain found himself and the 20th Maine at the far left end of the entire Union line. He quickly understood the strategic significance of the small hill, and the need for the 20th Maine to hold the Union left at all costs. The men from Maine waited until troops from the 15th Alabama Infantry regiment, under Col. William C. Oates, charged up the hill, attempting to flank the Union position. Time and time again the Confederates struck, until the 20th Maine was almost doubled back upon itself. With many casualties and ammunition running low, Col. Chamberlain recognized the dire circumstances and ordered his left wing (which was now looking southeast, compared to the rest of the regiment, which was facing west) to initiate a bayonet charge. From his report of the day: "At that crisis, I ordered the bayonet. The word was enough." While battlefield conditions make it unlikely that many men heard Chamberlain's order, most historians believe he initiated the charge.
The 20th Maine charged down the hill, with the left wing wheeling continually to make the charging line swing like a hinge, thus creating a simultaneous frontal assault and flanking maneuver, capturing 101 of the Confederate soldiers and successfully saving the flank.This version of the battle was popularized by the book, "Killer Angels" and the movie, "Gettysburg" and there is a historical debate on the validity of this account. Chamberlain sustained two slight wounds in the battle, one when a shot hit his sword scabbard and bruised his thigh, and another when his right foot was hit by a spent bullet or piece of shrapnel. For his tenacity at defending Little Round Top he was known by the sobriquet Lion of the Round Top. Prior to the Battle, Chamberlain was quite ill, developing malaria and dysentery. Later, due to this illness, he was taken off active duty until he recovered.
In April 1864, Chamberlain returned to the Army of the Potomac and was promoted to brigade commander shortly before the Siege of Petersburg. There, in a major action on June 18, at Rives' Salient, Chamberlain was shot through the right hip and groin. Despite the injury, Chamberlain withdrew his sword and stuck it into the ground in order to keep himself upright to dissuade the growing resolve for retreat. He stood upright for several minutes until he collapsed and lay unconscious from loss of blood. The wound was considered mortal by the division's surgeon, who predicted he would perish; Chamberlain's incorrectly recorded death in battle was reported in the Maine newspapers, and Lt. Gen. Ulysses S. Grant gave Chamberlain a battlefield promotion to the rank of brigadier general after receiving an urgent recommendation on June 19 from corps commander Maj. Gen. Gouverneur K. Warren: "He has been recommended for promotion for gallant and efficient conduct on previous occasion and yesterday led his brigade against the enemy under most destructive fire. He expresses the wish that he may receive the recognition of his services by promotion before he dies for the gratification of his family and friends." Not expected to live, Chamberlain displayed surprising will and courage, and with the support of his brother Tom, was back in command by November. Although many, including his wife Fanny, urged Chamberlain to resign, he was determined to serve through the end of the war.
In early 1865, Chamberlain was given command of the 1st Brigade of the 1st Division of V Corps, and he continued to act with courage and resolve. On March 29, 1865, his brigade participated in a major skirmish on the Quaker Road during Grant's final advance that would finish the war. Despite losses, another wound (in the left arm and chest), and nearly being captured, Chamberlain was successful and brevetted to the rank of major general by President Abraham Lincoln.
In all, Chamberlain served in 20 battles and numerous skirmishes, was cited for bravery four times, had six horses shot from under him, and was wounded six times.
On the morning of April 9, 1865, Chamberlain learned of the desire by Lee to surrender the Army of Northern Virginia when a Confederate staff officer approached him under a flag of truce. "Sir," he reported to Chamberlain, "I am from General Gordon. General Lee desires a cessation of hostilities until he can hear from General Grant as to the proposed surrender." The next day, Chamberlain was summoned to Union headquarters where Maj. Gen. Charles Griffin informed him that he had been selected to preside over the parade of the Confederate infantry as part of their formal surrender at Appomattox Court House on April 12.
Thus Chamberlain was responsible for one of the most poignant scenes of the Civil War. As the Confederate soldiers marched down the road to surrender their arms and colors, Chamberlain, on his own initiative, ordered his men to come to attention and "carry arms" as a show of respect. Chamberlain described what happened next:
Gordon, at the head of the marching column, outdoes us in courtesy. He was riding with downcast eyes and more than pensive look; but at this clatter of arms he raises his eyes and instantly catching the significance, wheels his horse with that superb grace of which he is master, drops the point of his sword to his stirrup, gives a command, at which the great Confederate ensign following him is dipped and his decimated brigades, as they reach our right, respond to the 'carry.' All the while on our part not a sound of trumpet or drum, not a cheer, nor a word nor motion of man, but awful stillness as if it were the passing of the dead.
Chamberlain's salute to the Confederate soldiers was unpopular with many in the North, but he defended his action in his memoirs, The Passing of the Armies. Many years later, Gordon, in his own memoirs, called Chamberlain "one of the knightliest soldiers of the Federal Army." Gordon never mentioned the anecdote until after he read Chamberlain's account, more than 40 years later.
Chamberlain left the army soon after the war ended, going back to his home state of Maine. Due to his immense popularity he served as Governor of Maine for four one-year terms after he won election as a Republican. His victory in 1866 set the record for the most votes and the highest percentage for any Maine governor by that time. He would break his own record in 1868. During his time in office he was attacked by those angered by his support for capital punishment and by his refusal to create a special police force to enforce the prohibition of alcohol.