Immanuel Kant wrote Perpetual Peace: A Philosophical Sketch in 1787. In this essay he argues for a League of Nations committed to the upholding of international law, to establish peace between nations. Three key elements appear: the state is to be viewed as a moral person; the earth is deemed to be originally owned by all; and perpetual peace ought to be the goal for humanity. The demand for universal peace is the final end of Kant’s doctrine of the Right -it is what reason dictates should be the proper condition of humanity, a universalisable categorical imperative, and hence all other acts should defer to it. This is an application of Kant’s third formula or principle of autonomy – act as though you were a law-maker in a kingdom of ends. However, in this article Roger Scruton argues that a  condition of perpetual peace is that member nations are republics. In the case of tyrannies like Saddam Husssein’s, there is still a requirement to go to war to protect the innocent, and Kantian ethics gives us a solid rational basis for a just war theory.

http://www.jrichardstevens.com/articles/scruton-kant.pdf

See also this excellent summary of Kant’s argument.

http://www.alexander-moseley.me.uk/Articles/just%20war%20theory/philosophers/Kant.htm