Clausula rebus sic stantibus
Clausula rebus sic stantibus 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).
Source: Wikipedia — Clausula rebus sic stantibus (CC BY-SA 4.0)