Rationalist theories of war assume that both sides to a potential war are rational, which is to say that each side wants to get the best possible outcome for itself for the least possible loss of life and property to its own side. Given this assumption, if both countries knew in advance how the war would turn out, it would be better for both of them to just accept the post-war outcome without having to actually pay the costs of fighting the war. This is based on the notion, generally agreed to by almost all scholars of war since Carl von Clausewitz, that wars are reciprocal, that all wars require both a decision to attack and also a decision to resist attack. Rationalist theory offers three reasons why some countries cannot find a bargain and instead resort to war: issue indivisibility, information asymmetry with incentive to deceive, and the inability to make credible commitments.[34]
Issue indivisibility occurs when the two parties cannot avoid war by bargaining because the thing over which they are fighting cannot be shared between them, only owned entirely by one side or the other. Religious issues, such as control over the Temple Mount in Jerusalem, are more likely to be indivisible than economic issues.
A bigger branch of the theory, advanced by scholars of international relations such as Geoffrey Blainey, is the problem of information asymmetry with incentives to misrepresent. The two countries may not agree on who would win a war between them, or whether victory would be overwhelming or merely eked out, because each side has military secrets about its own capabilities. They will not avoid the bargaining failure by sharing their secrets, since they cannot trust each other not to lie and exaggerate their strength to extract more concessions. For example, Sweden made efforts to deceive Nazi Germany that it would resist an attack fiercely, partly by playing on the myth of Aryan superiority and by making sure that Hermann Göring only saw elite troops in action, often dressed up as regular soldiers, when he came to visit.
Intelligence gathering may sometimes, but not always, mitigate this problem. For example, the Argentinian dictatorship knew that the United Kingdom had the ability to defeat them, but their intelligence failed them on the question of whether the British would use their power to resist the annexation of the Falkland Islands.
The American decision to enter the Vietnam War was made with the full knowledge that the communist forces would resist them, but did not believe that the guerrillas had the capability to long oppose American forces.
Thirdly, bargaining may fail due to the states' inability to make credible commitments.[35] In this scenario, the two countries might be able to come to a bargain that would avert war if they could stick to it, but the benefits of the bargain will make one side more powerful and lead it to demand even more in the future, so that the weaker side has an incentive to make a stand now.
Rationalist explanations of war can be critiqued on a number of grounds. The assumptions of cost-benefit calculations become dubious in the most extreme genocidal cases of World War II, where the only bargain offered in some cases was infinitely bad. Rationalist theories typically assume that the state acts as a unitary individual, doing what is best for the state as a whole; this is problematic when, for example, the country's leader is beholden to a very small number of people, as in a personalistic dictatorship. Rationalist theory also assumes that the actors are rational, able to accurately assess their likelihood of success or failure, but the proponents of the psychological theories above would disagree.
Rationalist theories are usually explicated with game theory, for example, the Peace War Game, not a wargame as such, rather a simulation of economic decisions underlying war
