The most ambitious treatise on war in English since Quincy Wright's Study of War appeared in 1942. The method is not philosophical or technical but historical, empirical and, in the author's phrase, sociological. He divides world history (the focus is mainly, but not only, on Europe) since medieval times into five ages: of dynasties (1400-1559), of religions (1559-1648), of sovereignty (1648-1789), of nationalism (1789-1917) and of ideology (1917-present). He then takes up, for each period, the key aspects of war as a social phenomenon (issues, motives, decisions, calculus of gains and costs, and beliefs), all from the standpoint of those making the vital choices for war or peace. It is all logical enough, perhaps too much so since, as the author concedes, many of these things are unmeasurable and one can inquire endlessly into the "causes" of war. Here the same wars and the same explanations keep turning up; thus, much of the text, for the reader, is déjà lu. The conclusions are well drawn, realistic and not wholly pessimistic.