Apart from the fact that it is listed together with good faith among the
universally recognized principles (preambular para. 3), this rule is also
enshrined in Article 26:
"Every treaty in force is binding upon the parties to it and must be
performed by them in good faith."
In Paul Reuter's words, this principle can be translated by the following formula: treaties "are what the authors wanted them to be and only what they wanted them to be and because they wanted them to be the way they are".
A party is not authorized to invoke the provisions of its internal law as
justification for its failure to perform a treaty (Article 27). Generally
speaking, this solid legal link is nor even weakened in the case severance of diplomatic relations between the parties to a given treaty (Article 63). The only limit to the "pacta sunt servanda" rule is to be found in the notion of "peremptory norm of general international law" (or jus cogens).
But apparently States expect increasingly out of realism that the treaties they conclude in certain areas, in particular with regard to the protection of the environment, will not be properly implemented by all States parties just out of respect for the "pacta sunt servanda" rule. This is why several recent treaties contain obligations to cooperate in order to facilitate compliance with the treaty obligations (see also Article 8 of the Ottawa Convention on the prohibition of landmines).
