George C. Marshall shaped a doctrine that placed decisive offensive action at the center of American military thinking. Defense was temporary. Offense restored initiative, broke the enemy’s cohesion, and brought victory. That philosophy became the foundation of United States Army doctrine and remains at its core to this day. FM 100-5, published on 15 June 1944 at the height of the Second World War, captured that philosophy in its clearest and most influential form. This wartime edition of Field Service Regulations stands as the pinnacle of the doctrine developed under Marshall’s leadership. It defined how American forces organized, maneuvered, coordinated, and fought across every theater of the war. The manual’s pages, introduced by Marshall himself, present a complete view of the Army’s approach to modern combined arms operations at a time when these principles were being tested on battlefields from Normandy to the Pacific. FM 100-5 explained that victory depended on unified effort. Infantry, armor, artillery, air power, engineers, and logistics had to act together as a single, flexible system. Commanders were expected to understand the capabilities of each arm and apply them with imagination, speed, and precision. At the same time, the manual rejected rigid formulas. It taught leaders to adapt, to avoid predictable patterns, and to use movement and firepower to keep the enemy off balance. Inside this volume, readers will find the full 1944 text on organization, reconnaissance, security, offensive and defensive operations, pursuit, river crossings, mountain warfare, jungle combat, airborne operations, and more. Every chapter reflects the combat lessons learned in North Africa, Sicily, Italy, and the Pacific, refined into doctrine that shaped the campaigns of 1944 and 1945. This volume is part of Milstar Books’ expanding WWII Special Collection, dedicated to preserving rare and historically significant military manuals from the Second World War. For historians, military professionals, reenactors, and students of strategy, this reprint offers direct access to one of the most important doctrinal manuals ever published. FM 100-5 remains a cornerstone of American military thought and an essential reference for understanding how the United States Army fought and won the largest war in its history.