Pacta sunt servanda it's the principle of mandatory force covering contracts signed between two or more parties. It consists of the idea that what is established in the contract and signed by the parties must be fulfilled. This is a Latin expression and means "covenants must be respected" or "agreements must be kept", in Portuguese.
The basic principle of pacta sunt servanda says that what is written becomes law between the parties who signed such a document. In this way, you cannot force someone to fulfill a contract to which they are not a signatory.
O pacta sunt servanda it is also a basic principle of Civil Law and International Law. This condition guarantees the legal security and autonomy of the parties when entering into such a contract.
Pacta sunt servanda and Rebus sic stantibus
Both principles govern the fulfillment of contracts, whether private or public.
According to the clause of the rebus sic stantibus, which represents the Unpredictability Theory, the mandatory contractual rules must apply as long as the conditions existing at the time of signing the contract remain the same over time.
In Latin, the expression rebus sic stantibus literally means "things being like this" or "while things are like this", in Portuguese.
Read more about Rebus sic stantibus.
If the characteristics that define the content of the contract change, there is no longer the obligation of the principle pacta sunt servanda in this.
In the case of a continuing benefits contract, for example, if a fact arises during its execution unforeseen event that makes it impossible for one of the parties to comply with the rules, the theory of unpredictability (rebus sic stantibus), with the aim of re-establishing the balance between the signatories.
See also what it means Contract termination.