Clausula rebus sic stantibus (Latin for "things thus standing") is the legal doctrine allowing for a contract or a treaty to become inapplicable because of a fundamental change of circumstances. In public international law the doctrine essentially serves an "escape clause" to the general rule of pacta sunt servanda (promises must be kept).
The doctrine is part of customary international law but is also provided for in the 1969 Vienna Convention on the Law of Treaties, under Article 62 (Fundamental Change of Circumstance).
https://en.wikipedia.org/wiki/Clausula_rebus_sic_stantibus
Vienna Convention on the Law of Treaties
Article 62 — Fundamental change of circumstances
1. A fundamental change of circumstances which has occurred with regard to those existing at the time of the conclusion of a treaty, and which was not foreseen by the parties, may not be invoked as a ground for terminating or withdrawing from the treaty unless:
(a) the existence of those circumstances constituted an essential basis of the consent of the parties to be bound by the treaty; and (b) the effect of the change is radically to transform the extent of obligations still to be performed under the treaty.
2. A fundamental change of circumstances may not be invoked as a ground for terminating or withdrawing from a treaty:
(a) if the treaty establishes a boundary; or (b) if the fundamental change is the result of a breach by the party invoking it either of an obligation under the treaty or of any other international obligation owed to any other party to the treaty.
3. If, under the foregoing paragraphs, a party may invoke a fundamental change of circumstances as a ground for terminating or withdrawing from a treaty it may also invoke the change as a ground for suspending the operation of the treaty.
Article 63 — Severance of diplomatic or consular relations
The severance of diplomatic or consular relations between parties to a treaty does not affect the legal relations established between them by the treaty except in so far as the existence of diplomatic or consular relations is indispensable for the application of the treaty.
Article 64 — Emergence of a new peremptory norm of general international law (jus cogens)
If a new peremptory norm of general international law emerges, any existing treaty which is in conflict with that norm becomes void and terminates.
JUS COGENS
Jus cogens (from Latin: compelling law; from English: peremptory norm) refers to certain fundamental, overriding principles of international law.
Article 53 of the Vienna Convention on the Law of Treaties (VCLT): “[A] treaty is void if, at the time of its conclusion, it conflicts with a peremptory norm of general international law.”
Examples of jus cogens norms include prohibitions against crimes against humanity, genocide, and human trafficking.
