Given the weight of the question it clearly cannot be settled merely in a posting concerning the fifth preliminary article of Perpetual Peace but will require considerable attention, both in relation to the working out of general principles and also in attending to the numerous complexities of given particular examples. The fifth article of Perpetual Peace is controversial in its interpretation precisely due to its connection with such larger issues.
In looking at this fifth preliminary article it is useful also to look at the context of Kant's writing of the whole work. Perpetual Peace was published in 1795, the same year in which Prussia withdrew from a coalition against revolutionary France and concluded a separate peace with the republic. It is against the backdrop of the revolutionary wars that Kant frames his fifth preliminary article. The article states that: "No state shall forcibly interfere in the constitution and government of another state" (Ak. 8: 346). The principle simply as stated thus indicates that there is no ground for alteration of the political affairs of another state. However, despite the fact that the 5th article seems to assert something fairly authoritative, whatever one's view of the principle in question, reading Kant's discussion of the article suggests instantly that in fact things are complicated.
Kant goes on to specify that things are different if the state in question has already split into two with two separate elements laying claim to the whole. It is different, that is, in cases where a civil war has sundered the pre-existent state. Since this was clearly the case with the Spanish Civil War (and is one way in which the break-up of Yugoslavia could be described) it follows that in such a case Kant's preliminary article does not apply. The situation in which such civil war is taking place is, Kant declares, "anarchy" and so it is not interference for a foreign state to give assistance to one of the warring parties.
Having introduced this mitigation of the original principle Kant proceeds to mitigate the mitigation. The next point is that for the foreign state to be able to intervene in a way that is not interference requires that the internal conflict be "critical" as otherwise such intervention would make the autonomy of all states insecure. The judgment in question might well be thought to have been the basis of the coalition formed against Saddam Hussein's Iraq in the 1st Gulf War since Kuwait was here forcibly incorporated into a neighbouring state despite not itself being internally disrupted. However whilst this mitigation of the mitigation might initially appear helpful it clearly introduces an important problem in its wake. When the foreign state is making a decision concerning such intervention without wishing to be accused of "interference" how is it to decide whether or not the situation is "critical" enough to justify it? No criteria are given in Kant's discussion of the 5th preliminary article. As we will see subsequently the basic principle of the 5th preliminary article is also not evidently in harmony with other elements of Kant's discussion.