This dissertation explores the development of America’s infantry forces between 1918-1945. While doing so, it challenges and complicates the traditional narrative that highlights the fierceness of the rivalry between the U.S. Army and Marine Corps. During the First World War, both commissioned and enlisted Marines attended U.S. Army schools and served within Army combat formations, which brought the two closer together than ever before. Both services became bonded by a common warfighting paradigm, or way of battle, that centered upon the infantry as the dominant combat arm. All other arms and services were subordinated to the needs and requirements of the infantry. Intelligent initiative, fire and maneuver by the smallest units, penetrating hostile defenses while bypassing strong points, and aggressive, not reckless, leadership were all salient characteristics of that shared infantry way of battle. After World War I, Army and Marine officers constructed similar intellectual proposals concerning the ways to fight the next war. Although there were differences in organizational culture, the two were more alike in their respective values systems than historians have realized. There was mutual admiration, and targeted attempts to replicate each other’s combat thinking and spirit. They prepared for battle by observing each other’s doctrine, and sharing each other’s conception of modern combat. When preparation turned to execution in World War II, they created solutions for battlefield problems that evolved from their near-identical way of battle. At the conclusion of the war, the common bonds between the Army and Marine Corps were all but forgotten. This, ultimately, led to increased friction during the Congressional defense unification battles in 1946. / History