Formal alliances, 1815—1939

Abstract Although students of world politics have come increasingly to speak of their empirical domain as a social system, little effort has gone into the systematic observation and measurement of its formal or informal structure. One structural attribute of the inter national system which leaves a definite trace and is therefore readily measurable, is that of alliance patterns. This paper examines all written (peacetime only) alliances among system members, describes the procedures for classifying them, and then transforms such information into quantitative indicators of alliance aggregation for various forms of the international system for every year between 1815 and 1939. Such data make it possible to treat this particular system attribute as either a dependent or an inde pendent variable for purposes of hypothesis testing or exploration in world politics.