At the beginning of the American Civil War, 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 Maine's Governor Israel Washburn, Jr., "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 assigned to the 3rd Brigade, 1st Division, 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. The 20th Maine fought at the 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 1863 due to an outbreak of smallpox in their ranks (which was caused by an errant smallpox vaccine), keeping them on guard duty in the rear. Chamberlain was promoted to colonel of the regiment in June 1863 upon the promotion of Ames.