Kant's claim that liberal states are pacific in their international relations with other liberal states was revived in the 1980s. In a much-cited article, Michael Doyle argued that liberal states have created a separate peace.

According to Doyle, there are two elements to the Kantian legacy, restraint among liberal states and international imprudence in relations with non-liberal states. Although the empirical evidence seems to support the democratic peace thesis, it is important to bear in mind the limitations of the argument.

In the first instance, for the theory to be compelling, believers in the thesis need to provide an explanation as to why war has become unthinkable between liberal states. Kant had argued that if the decision to use force were taken by the people, rather than by the prince, then the frequency of conflicts would be drastically reduced. But, logically, this argument also implies a lower frequency of conflicts between liberal and non-liberal states, and this has proven to be contrary to the historical evidence.

An alternative explanation for the democratic peace thesis might be that liberal states tend to be wealthy, and therefore have less to gain and more to lose by engaging in conflicts than poorer authoritarian states. Perhaps the most convincing explanation of all is the simple fact that liberal states tend to be in relations of amity with other liberal states.