This work traces the evolution of two unique U.S. Army organizations - divisions and brigades - which combined combat arms, combat support, and combat service support units into well-oiled engines for war. The Army has used divisions for over two hundred and twenty years on the battlefield and for nearly eighty years has maintained them in peacetime as well. Both organizations have played a pivotal role in the American military experience, and their exploits form the core of the Army's history in the twentieth century. This study is a systematic account of the way these two organizations evolved, highlighting the rationales behind that evolution and the many factors that played a part in bringing those changes into reality. In this work the reader, whether military or civilian, can follow the development of two of the Army's complex organizations. Force planners today will find the challenges faced by their predecessors in making these institutions responsive to an ever-changing threat in an evolving political and technological environment highly relevant. By telling this story in a comprehensive manner, the volume makes a significant contribution to the history of the Army.