The Motivation of Chamberlain

Joshua Chamberlain was commissioned a Lieutenant Colonel in the 20th Maine Infantry Regiment during the Civil War. He took command of the Regiment in 1863. Chamberlain received the Medal of Honor for leading a bayonet charge during the Battle of Gettysburg that drove the Confederates from the extreme left of the Union lines at Little Round Top.
In this video clip, with two- thirds of his Regiment killed or wounded, Chamberlain takes possession of a group of Union deserters. Shortly before the Regiment moves out to fight at Gettysburg, Chamberlain invites the deserters to join his outfit and fight in the upcoming battle. He doesn't talk about the duty they owe their country. Chamberlain discusses the purpose of this cause, to set other men free. He talks about the idea that all men have value, and the purpose of this campaign is to fight for each other.  
Listen to the speech Jeff Bridges, who portrays Lt Colonel Chamberlain, gives to the deserters in the movie Gettysburg: