[A review of history professor's Gary Gallagher's "The Union War."] The question raised by Gary Gallagher in his important work "The Union War" centers around the issue of the reasons for fighting the war. Focusing on the Union side, this talented author, who teaches at the University of Virginia, looks to original sources to find out just what the general population and the members of the military thought the war was all about. Answers do not come easy.