The outbreak of World War II set in motion a massive expansion of the United States Marine Corps, leading to a 24-fold increase in size by August 1945. This book is the first of several volumes to examine the Corps's meteoric wartime expansion and the evolution of its units. It covers the immediate pre-war period, the rush to deploy defense forces in the war's early months, and the Marines' first combat operations on Guadalcanal, New Georgia, and Bougainville. It focuses on the 1st, 2d, and 3d Marine Divisions (MarDivs) and the provisional 1st, 2d, and 3d Marine Brigades (MarBdes).