This is a course of six lectures showing the influence on the art of war of the campaigns of Alexander, Hannibal, Caesar, Gustavus Adolphus, Frederick, and Napoleon. These lectures were delivered in Boston, under the auspices of the Lowell Institute, in January, 1889. There conciseness needs but the apology of scant time. Little can be said about Alexander or Napoleon within the limit of an hour. The sketches are of necessity meager. They are a summary in part of a larger work, of which the author hopes soon to begin the publication, in which a volume will be devoted to each great captain, and mention made of other soldiers who have contributed to the growth of the art of war. The lectures aim to indicate briefly what we owe to the great captains, and to draw an intelligible outline of their careers, which may be filled in by reference to the extended narratives of others. Historical detail often assumes prominence in the mind to the exclusion of general form. It is the latter which it is attempted to portray.