Summi Pontificatus §77

⁷⁷ Now, it is true that with the passage of time and the substantial change of circumstances, which were not and perhaps could not have been foreseen in the making of a treaty, such a treaty or some of its clauses can in fact become, or at least seem to become unjust, impracticable or too burdensome for one of the parties. It is obvious that should such be the case, recourse should be had in good time to a frank discussion with a view to modifying the treaty or making another in its stead. But to consider treaties on principle as ephemeral and tacitly to assume the authority of rescinding them unilaterally when they are no longer to one's advantage, would be to abolish all mutual trust among States. In this way, natural order would be destroyed and there would be seen dug between different peoples and nations trenches of division impossible to refill.