Immanuel Kant argued that nations, like individuals, need an established form of rule by law, which can only be created by a voluntarily assembled “league of nations”, what we now call the United Nations. If nations do not have a system of law, and a means of determination of judgement, then they resort to force (war) to settle disputes even though victory in war doesn’t necessarily guarantee a right and just settlement will always be achieved. Therefore, this federation of free states serves as a forum for these nations to come together and peacefully negotiate, providing an additional alternative to war.
In Perpetual Peace: A Philosophical Sketch, Kant’s proposal for the creation of a Federation of Free States is followed by an explanation that this would not resemble an international state. The argument is that “a number of nations forming one state would constitute a single nation”, which contradicts the right of nations in relation to one another.