Wikipedia
Pacta sunt servanda ( Latin for "agreements must be kept"), a brocard, is a basic principle of civil law, canon law, and international law.
In its most common sense, the principle refers to private contracts, stressing that contained clauses are law between the parties, and implies that nonfulfillment of respective obligations is a breach of the pact.
In civil law jurisdictions this principle is related to the general principle of correct behavior in commercial practice — including the assumption of good faith — is a requirement for the efficacy of the whole system, so the eventual disorder is sometimes punished by the law of some systems even without any direct penalty incurred by any of the parties. However, common law jurisdictions usually do not have the principle of good faith in commercial contracts, therefore it is inappropriate to state that pacta sunt servanda includes the principle of good faith.
With reference to international agreements, "every treaty in force is binding upon the parties to it and must be performed by them in good faith." Pacta sunt servanda is related to good faith, while pacta sunt servanda does not equate with good faith. This entitles states to require that obligations be respected and to rely upon the obligations being respected. This good faith basis of treaties implies that a party to the treaty cannot invoke provisions of its municipal (domestic) law as justification for a failure to perform. However, with regards to the Vienna Convention and the UNIDROIT Principles it should be kept in mind that these are heavily influenced by civil law jurisdictions. To derive from these sources that pacta sunt servanda includes the principle of good faith is therefore incorrect.
The only limit to pacta sunt servanda are the peremptory norms of general international law, called jus cogens (compelling law). The legal principle clausula rebus sic stantibus, part of customary international law, also allows for treaty obligations to be unfulfilled due to a compelling change in circumstances.