This book examines the organizational development, mobilization, deployment and combat actions of World War II US Army infantry divisions up until the end of 1943. As John J Sayen Jr shows, the US Army infantry division was an intricate system of men and equipment welded together by doctrine and organization into an entity that could fight, maneuver, communicate within itself as well as with outside entities, and regenerate itself through a supply and replacement system. Among the units covered are the separate infantry regiments and battalions of the "standard" type, showing how the Army's new "triangular" infantry division (based on three infantry regiments) evolved from the earlier "square" division (based on four).