This book examines the development of the American Army corps structure during World War I. The corps formation developed in World War I by General John J. Pershing was the culmination of 56 years of lessons learned from the Civil War through the beginning of World War I. The success of the American Expeditionary force (AEF) in World War I was, in large part, the result of the development and combat effectiveness of the corps formation. The corps formation allowed commanders flexibility, concentrated firepower, sustainability, and increased command and control. The American corps formation in World War I led the AEF to victory in the Saint Mihiel and Meuse-Argonne campaigns. The American corps formation provided resources and capabilities that helped provide the needed edge to defeat the Central Powers and help end World War I.