The period covered in this volume, from about 1850 until the dawn of the twenty-first century, was the most belligerent in human history. More people were involved in the preparation and prosecution of warfare, and more men, women, and children fell victim to military violence than in all previous eras put together. The period since 1850 is the era of modern war. It can be considered under three headings, as the age of mass, the age of machines, and the age of management.

The age of mass is the principal subject of Part I of this volume. The dramatic growth of military capacity involved creating and sustaining not only the largest armies in history, but also the most complex and demanding societies. Size and complexity were products of the Industrial Revolution. Developments in farm technology and steam transportation created a global agricultural revolution, which enabled large-scale transfers of young men to barracks and battlefields. Weapons and uniforms became items of mass production. Telephones, telegraphs, and typewriters made increased control possible, keeping mass armies from becoming armed hordes.