In49b.c., Julius Caesarhalted his army on the banks of the Rubicon River in northern Italy. Accordingto Suetonius, he paused in momentary hesitation, before sweeping across thewaters toward Rome with the immortal phrase Alae iacta est (The die has beencast).1By violating an ancient Roman law forbidding any general to cross theRubicon with an army, Caesar’s decision made war inevitable. Ever since,“crossing the Rubicon” has come to symbolize a point of no return, when thetime for deliberation is over and action is at hand.In this article we set out the Rubicon theory of war. When people be-lieve they have crossed a psychological Rubicon and perceive war to be im-minent, they switch from what psychologists call a “deliberative” to an“implemental” mind-set, triggering a number of psychological biases, mostnotably overconªdence.2These biases can cause an increase in aggressive orrisky military planning. Furthermore, if actors believe that war is imminentwhen it is not in fact certain to occur, the switch to implemental mind-sets canbe a causal factor in the outbreak of war, by raising the perceived probabilityof military victory and encouraging hawkish and provocative policies.The Rubicon theory of war has several important implications for interna-tional relations theory and practice. First, it helps to resolve a major paradox ininternational relations: the widespread fear and anxiety that underlies the se-curity dilemma in times of peace and the prevalence of overconªdence on theDominic D.P. Johnson is Reader in Politics and International Relations at the University of Edinburgh.Dominic Tierney is Assistant Professor of Political Science at Swarthmore College.The authors would like to thank Peter Gollwitzer, Ellen Langer, Jack Levy, Rose McDermott, AmyOakes, Stephen Peter Rosen, Shelley Taylor, Bradley Thayer, and Richard Wrangham for com-ments on work leading to this manuscript. An early version of the paper was presented as “TheTipping Point for War: Overconªdence and the Certainty of Conºict” at the 2007 annual conven-tion of the International Studies Association in Chicago, Illinois, and the authors thank Il HyunCho, Tara Lavallee, and Jessica Weeks for their constructive suggestions. Finally, they thank theanonymous reviewers for extremely thorough and helpful comments.1. C. Suetonius Tranquillus, The Lives of the Twelve Caesars, Vol. 1: Julius Caesar (Cambridge, Mass.:Harvard University Press, 1989).2. Peter M. Gollwitzer, “Mindset Theory of Action Phases,” in Paul A.M. Van Lange, Arie W.Kruglanksi, and E. Tory Higgins, eds., Handbook of Theories of Social Psychology, Vol. 1 (London:Sage, 2011), pp. 526–545. “Mind-set” refers to characteristic cognitive orientations, which representthe sum total of activated cognitive procedures.The Rubicon Theory of WarThe Rubicon Theoryof WarDominic D.P. JohnsonandDominic TierneyHow the Path to Conºict Reaches thePoint of No ReturnInternational Security, Vol. 36, No. 1 (Summer 2011), pp. 7–40© 2011 by the President and Fellows of Harvard College and the Massachusetts Institute of Technology.7eve of war.3Part of the explanation may be that actors experience a signiªcantswitch in mind-set as conºict draws near.Second, the Rubicon theory advances the debate about whether leaders orstates are rational actors in international politics.4If rationality depends onmind-sets, then the accuracy of the rational actor model hinges on when dur-ing a crisis one looks for evidence. Early on in the decisionmaking process, aleader is more likely to be in a deliberative mind-set and may approximatea rational actor. Later during the crisis, the same leader is more likely to be inan implemental mind-set, and may display a range of biases that deviate fromrationality.Third, the study contributes to the growing literature on the role of psychol-ogy in international relations. Scholars have proposed a wide range of psycho-logical factors as causes of war, including glory, pride, honor, reputation,revenge, justice, fear, and hatred, as well as numerous biases in judgment anddecisionmaking.5Here, we focus on the role of shifting mind-sets in militaryplanning and the outbreak of war. Because mind-sets are a master lever thatsuppresses or ampliªes a range of associated psychological biases, the Rubicontheory of war distills what can seem like a laundry list of biases into a coher-ent, systematic, and testable pattern. Implemental mind-sets cause several dif-ferent biases to push judgment and decisionmaking in the same direction,promoting overconªdence.Fourth, the Rubicon theory contributes to a long-standing research programlinking overconªdence and war. In two landmark studies a quarter of a cen-tury apart, Geoffrey Blainey argued that overconªdence (or “false optimism”)is “a potent and pervasive cause of war,” and Stephen Van Evera found it “cru-cial to an understanding of war.”6Overconªdence can lead decisionmakers toInternational Security 36:1 83. Robert Jervis, “Cooperation under the Security Dilemma,” World Politics, Vol. 30, No. 2 (January1978), pp. 167–214; and Geoffrey A. Blainey, The Causes of War (New York: Free Press, 1973).4. James D. Fearon, “Rationalist Explanations for War,” International Organization, Vol. 49, No. 3(Summer 1995), pp. 379–414; and Jonathan Mercer, “Rationality and Psychology in InternationalPolitics,” International Organization, Vol. 59, No. 1 (Winter 2005), pp. 77–106.5. Robert Jervis, Perception and Misperception in International Politics (Princeton, N.J.: Princeton Uni-versity Press, 1976); Rose McDermott, Political Psychology in International Relations (Ann Arbor:University of Michigan Press, 2004); David O. Sears, Leonie Huddy, and Robert Jervis, eds., OxfordHandbook of Political Psychology (Oxford: Oxford University Press, 2003); Jack S. Levy and WilliamR. Thompson, Causes of War (Oxford: Wiley-Blackwell, 2010); Monica Duffy Toft, The Geography ofEthnic Violence: Identity, Interests, and the Indivisibility of Territory (Princeton, N.J.: Princeton Univer-sity Press, 2003); Jonathan Mercer, Reputation and International Politics (Ithaca, N.Y.: Cornell Univer-sity Press, 1996); and David A. Welch, Justice and the Genesis of War (Cambridge: CambridgeUniversity Press, 1993).6. Blainey, The Causes of War, p. 35; and Stephen Van Evera, Causes of War: Power and the Roots ofConºict (Ithaca, N.Y.: Cornell University Press, 1999), p. 16. See also Dominic D.P.