Book excerpt: Almost immediately after Abraham Lincoln issued the Emancipation Proclamation, abolitionists began to call for the raising of black regiments. The South and most of the North responded with outrage. Southerners vowed to enslave black soldiers captured in battle, while many northerners claimed that blacks lacked the courage to fight. Yet Boston's Brahmins, always eager for a moral crusade, launched one of the greatest experiments in American history. In Thunder at the gates, Douglas R. Egerton chronicles the formation and exploits of the 54th and 55th Massachusetts Infantry and the 5th Massachusetts Cavalry -- regiments led by whites but composed of black men born free or into slavery.