Pacta sunt servanda (VCT Article 26): "Every treaty in force is binding upon the parties to it and must be performed by them in good faith" — the system's constitutional rule: the sanctity of agreements; no party may unilaterally renege (Article 27's companion: internal law is no excuse).
Jus cogens (VCT Articles 53 and 64): the peremptory norms of general international law — accepted and recognized by the international community as a whole, from which no derogation is permitted: the prohibitions of aggression, genocide, slavery, torture, the basic rules of humanitarian law, self-determination. A treaty conflicting with jus cogens is void ab initio (Article 53); a new emerging norm destroys existing conflicting treaties (Article 64 — Article 53's forward twin). The doctrines' balance: pacta's stability qualified only by the community's highest norms — obligations erga omnes their procedural cousin (Barcelona Traction — PIL-I).
Rebus sic stantibus (VCT Article 62 — "the circumstances remaining so"): the fundamental change of circumstances doctrine: a treaty's termination/withdrawal where the
change (i) was unforeseen, (ii) relates to an essential basis of the parties' consent, and (iii) transforms the extent of obligations still to be performed. The strict conditions (the Article's own bars): never for boundary treaties (the stability of borders); never invoked by a party whose own breach caused the change. The doctrine's function: pacta's safety valve against impossibility-in-substance — narrowly construed (the Gabčíkovo case's disciplined application).