Early US History American Journey Essay

Can you help me understand this History question? Book:The American Journey: A History of the United States, Volume 1 7th Edition, 2014ISBN-13: 978-0205960965 David Goldfield, Carl Abbott, Virginia Dijon Anderson, Jo Ann E. Argersinger, Peter H. Argersinger, and William BarneyQuestion:What were the vital factors that led to a Union victory in the Civil War?