Clausula rebus sic stantibus

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).
Because the doctrine is a risk to the security of treaties, as its scope is relatively unconfined, the conditions in which it may be invoked must be carefully noted.
Function in international law.
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). Although the doctrine is not mentioned by name, Article 62 provides the only justifications for its invocation: the circumstances that existed at the time of the conclusion of the treaty were indeed objectively essential to the obligations of treaty (sub-paragraph A), and the instance for the change of circumstances has had a radical effect on the obligations of the treaty (sub-paragraph B).
If the parties to a treaty had contemplated for the occurrence of the changed circumstances, the doctrine does not apply and the provision remains in effect. "Clausula rebus sic stantibus" relates to changed circumstances only if they had never been contemplated by the parties. That principle is clarified in the "Fisheries Jurisdiction" Case ("United Kingdom v. Iceland", 1973).
Although it is clear that a fundamental change of circumstances might justify terminating or modifying a treaty, the unilateral denunciation of a treaty is prohibited. A party does not have the right to denounce a treaty unilaterally. (However, that has been debated.)
Function in private law.
The principle of "clausula rebus sic stantibus" exists in all legal systems which descend from Roman law. In Swiss law, article 119 of the Swiss Code of Obligations is the source of the principle's applicability in Swiss contract law.
History.
A key figure in the formulation of "clausula rebus sic stantibus" was the Italian jurist Scipione Gentili (1563–1616), who is generally credited for coining the maxim "omnis conventio intelligitur rebus sic stantibus" ('every convention is understood with circumstances as they stand'). The Swiss legal expert Emer de Vattel (1714–1767) was the next key contributor. Vattel promoted the view that 'every body bound himself for the future only on the stipulation of the presence of the actual conditions' and so 'with a change of the condition also the relations originating from the situation would undergo a change'. During the 19th century, civil law came to reject the doctrine of "clausula rebus sic standibus", but Vattel's thinking continued to influence international law, not least because it helped reconcile 'the antagonism between the static nature of the law and the dynamism of international life'. While individual cases invoking the doctrine were much disputed, the doctrine itself was little questioned. Its provision in the 1969 Vienna Convention on the Law of Treaties established the doctrine firmly but not without dispute as 'a norm of international law'.