This is the final volume of a series devoted to the history of war, originally published in 1920. It considers new developments such as the use of gunpowder, the invention of firearms and the employment of cannons that shot stone and later iron balls. After reviewing the establishment of a European infantry, the author discusses the transformation of loose confederations of knights into cavalry, the organization of fighting mercenaries and the changing of mercenary bands into standing armies. It describes such personalities as Maurice of Orange, the father of military discipline; Gustavus Adolphus of Sweden, perfecter of infantry tactics; Oliver Cromwell, Frederick the Great and Napoleon Bonaparte.