Although Woodrow Wilson is the father of the Liberal School of International Relations, placing a lot of emphasis on the role of supranational institutions and connections between states, there really is a short answer in that the first proposal of some sort of true supra-national higher order (and not just, like, the reduction of multiple states to semi-independence by a more powerful state) comes from Immanuel Kant, who in 1795 proposed a "Federation of Free States." Although not identical (Kant was advising that states with codified legal systems, what he terms "Republican Constitutions," unite to ward off tyrannical foreign aggressors) the modern Liberal School of International Relations has its roots deeply embedded in Kantian ideas.