The basic claims of the democratic peace hypothesis are that democracies are unlikely to engage in wars with each other and that their existence generally promotes peace. However, the foremost exponent of the hypothesis, Michael W. Doyle, introduces a caveat that is often missing from discussion of it. This is to the effect that whilst democracies may be less likely to engage in wars with each other than with non-democracies they may in fact be generally aggressive towards non-democracies or overly wary of them. Doyle's research involves empirical discussion of a large number of wars dating from the early 19th century until today and includes some methodological claims that have been disputed. One of the problems with evaluating his claims includes the question of how one is to define when a "democracy" has been established. Included in the difficulty of this question is when a democracy can be regarded as stably in place. So some research, in opposition to the spirit,if not the letter, of Doyle's has claimed that "developing democracies" that are in transition from authoritarian rule to democratic governance are more likely to engage in warfare than settled authoritarian governments or democratic ones.
Allied to the claim concerning "developing democracies" is one concerning the relationship between "democracy" and the rule of law. A number of thinkers have set out the argument that the connection between democracy and peaceful intentions claimed by Doyle has been misinterpreted by him. The basis of this claim is that a state that has established a settled rule of law has created with this a basis for predicable behaviour externally as well as internally so that other states become less aggressive towards it and it, similarly, is more pacific in its relation to others. On the basis, it is then claimed, of the achieved rule of law and settled pacific relations, a transition to democracy becomes possible in a situation that guarantees continued peace. This argument would further suggest revision in both Doyle's original claim and the counter one concerning "developing democracies" suggesting that if a developing democracy emerges on the ground of a previous settled rule of law then it will tend towards pacific intentions and behaviour but not if otherwise.
Other historical problems abound, not least concerning the general tendency towards empire building by the UK during its development and the arguably aggressive policies of the US at different points of its history. Doyle gives a partial reply to these concerns in this article.
John Rawls also discusses Doyle's hypothesis in The Law of Peoples and describes it as "as close as anything we know to a simple empirical regularity in relations among societies". Despite this Rawls clearly points to behaviour by the US in which appeal to national security enables secretive decisions to conduct covert operations but suggests a convergence towards an asymptotic ideal will generally lead towards the pacific behaviour predicted in Doyle's hypothesis.
A divergence between the claims made by Doyle and the republican one of Kant is also apparent given that the democracies of today have large professional armies and are often funded through foreign debt so that they don't meet the conditions of the preliminary articles for perpetual peace. Doyle's own work, unlike that of some of his followers, is not one that supports the general view of perpetual peace being established between democracies and non-democracies however and, given the problems of ensuring that democracy develops in the right way and under the right conditions, is open to clear mitigation.