There is a burgeoning literature in international relations on war and its causes. It is supplemented by important works in history, sociology, psychology and economics. Most of the major studies of war by international relations scholars approach it from a realist perspective. They assume security is the principal concern of states and its absence the principal cause of war. Realist theories elaborate conditions (e.g. security dilemma, polarity, power transition) thought to be responsible for acute conflicts and mechanisms (e.g. military preparedness, alliances, the balance of power) expected to determine when they lead to war. Scholars working in the liberal paradigm are more interested in peace than war and have theorized its underlying conditions. The Democratic Peace, the flagship liberal research program on this topic, stipulates that democracies do not fight one another, although its proponents disagree among themselves about why this is so. It is a narrow claim, as Democratic Peace theorists acknowledge that democracies are no less warlike than other regimes. V. I. Lenin authored a Marxist theory of war. It assumes that economic interest is the principal driver of foreign policy, and attributes World War I to competition for markets and raw materials. In the last decade, rational theories of war have gained prominence. Most embody realist principles and assume that leaders are substantively and instrumentally rational.