Analytical studies of war can be traced back at least to the great work of the historian Thucydides, but systematic exploration of war as a unique but generic form of behavior between political communities was undertaken initially by political philosophers. Machiavelli, Rousseau, Kant, Hobbes, Hegel, and others had significant things to say about the etiology and consequences of war, but their insights were suggestive and prescriptive rather than empirical. They could enumerate the reasons wars are likely, but their causal statements were mostly hypothetical. Few had systematic evidence to support them.

The search for patterns and generalizations based on accumulated evidence is of more recent vintage. Today there is a large literature that has a common focus on the “causes of war.” It is not my purpose to examine in detail this important corpus of work, but it may be appropriate to reflect on some of its achievements and shortfalls because yet another book on war must be justified either as filling a gap or extending in significant ways existing bodies of knowledge.