This monograph examines military doctrine and explains why understanding its evolution and the influences that shape it are of vital importance to military practitioners, strategists, and statesmen alike. Doctrine, defined herein as the expression of a military's institutional 'belief system', constitutes a significant yet hitherto unrecognized means by which this belief system can be understood and evaluated. This understanding and evaluation is in turn important because it is this belief system that determines the way a military fights, the relationship it will have with the state and society that sustain it, and its institutional culture. To get the belief system right means good strategy, victory, stable civil-military relations, and organizational wellbeing. Getting it wrong means sub-optimal strategy and operational outcomes or even defeat, strained civil-military relationships, and organizational dysfunction. This is why it is vital that military practitioners, strategists, and statesmen all have a well-developed understanding of this belief system and its implications. Yet currently, many do so only subconsciously, if at all. The aim of this monograph is to help make this understanding explicit.