The book examines human gregariousness as an instinct grounded in biology and psychology, explaining how herd tendencies shape individual thought, emotions, and social institutions. It outlines mental traits produced by group life, the growth of collective prejudices and morale, and how acts of authority influence both external aims and national psychology. Drawing applications to wartime and peacetime, it explores mass behaviour under conflict, the fragility of civilization when social habits weaken, and psychological signs that can be anticipated. The author argues for a biologically informed psychology as a practical guide to social conduct and offers principles for more rational statecraft and postwar reconstruction.