80 With respect to States themselves, Our predecessors have constantly taught, and We wish to lend the weight of Our own authority to their teaching, that nations are the subjects of reciprocal rights and duties. Their relationships, therefore, must likewise be harmonized in accordance with the dictates of truth, justice, willing cooperation, and freedom. The same law of nature that governs the life and conduct of individuals must also regulate the relations of political communities with one another.
Source: Pacem in Terris (Vatican.va)