Why do the combat capabilities of individual soldiers vary so much? This book seeks to provide an answer to this and other questions about variability in combat performance. Some soldiers flee quickly from the battlefield, while others endure all hardships until the bitter end. Some combat units can perform numerous types of missions, while others cannot keep themselves organised during peacetime. Some militaries armed with obsolete weapons have outfought enemies with the latest weaponry, and some massively outnumbered armies have beaten back much more numerous opponents. In this first social scientific study of the effectiveness of combat troops, the author evaluates competing explanations for the varying combat capabilities and performances. The author assesses four main explanations, each emphasising the influence of a single factor. The first focuses on material endowments. How well funded, armed, and supplied are the troops? Secondly, do democracies produce better commanders, superior strategies, and more motivated and better-managed personnel; or are more authoritarian forms of government better at imposing military discipline? Thirdly, does giving more power to the troops on the ground in individual combat units enable them to be more flexible in fast-changing situations and circumstances? Fourthly, do soldier capabilities and performance reflect intrinsic attributes, such as prior civilian values? In the age-old nature vs. nurture argument, the author finds that intrinsic qualities do count, but that extrinsic factors, such as training and environment, matter even more.