Treaty Law: How Nations Make, Interpret, and Break International Agreements

The Rules That Govern the Rules

On May 23, 1969, the Vienna Convention on the Law of Treaties (VCLT) was adopted at a UN diplomatic conference in Vienna. It entered into force in 1980 and today has 116 state parties. Often called the "treaty on treaties," it provides the foundational rules for how international agreements between states are formed, interpreted, amended, and terminated. Even states that have not ratified the VCLT are bound by many of its provisions — because they reflect customary international law.

The VCLT governs treaties between states. Separate frameworks cover treaties between states and international organizations, and agreements between international organizations. But the VCLT's rules — on consent, interpretation, invalidity, and termination — are the architectural backbone of the entire international legal order. Every bilateral investment treaty, every arms control agreement, every trade deal, every UN Charter provision operates under rules the VCLT codifies.

What Makes Something a Treaty?

Under Article 2(1)(a) of the VCLT, a treaty is an international agreement concluded between states in written form and governed by international law, whether embodied in a single instrument or in two or more related instruments and whatever its particular designation. The definition has two critical features: it covers written agreements only (oral agreements are not treaties under the VCLT) and it requires governance by international law (not domestic law).

The designation does not determine legal status. An agreement called a "treaty," "convention," "protocol," "charter," "covenant," "exchange of notes," or "memorandum of understanding" may or may not be a legally binding treaty depending on the parties' intent and the content of the instrument. Many MOUs are deliberately drafted to avoid treaty status, providing political commitments without legal obligation. The distinction matters enormously in practice.

From Signature to Ratification: The Consent Journey

A state becomes bound by a treaty through a multi-stage process involving distinct legal acts, each with different effects.

• Negotiation: State representatives negotiate the text, often over years or decades. Negotiators act under full powers — formal authorization to negotiate, adopt, and authenticate the treaty text.
• Adoption: States agree on the final text. For multilateral treaties, this typically requires a two-thirds majority of states participating in negotiations.
• Authentication: The treaty text is established as definitive and authentic, typically through signature by negotiating representatives.
• Signature: Signing a treaty does not make it binding. It does create an obligation to refrain from acts that would defeat the treaty's object and purpose (Article 18 VCLT) — a significant interim obligation.
• Ratification: The formal act by which a state expresses its consent to be bound. In most constitutional systems, ratification requires domestic legislative approval before the executive can submit the ratification instrument to the treaty depositary.
• Entry into force: A treaty enters into force when the conditions specified in it are met — typically when a minimum number of states have ratified.

The US Senate's two-thirds ratification requirement for treaties (Article II, Section 2 of the Constitution) is among the most demanding in the world and has resulted in numerous agreements being submitted as executive agreements — which require only majority congressional approval or, in some cases, no congressional approval at all. The distinction between treaties and executive agreements under US domestic law has no counterpart in international law, which treats both as binding international obligations.

Reservations: Modifying Treaty Obligations

A reservation is a unilateral statement made by a state when signing, ratifying, or acceding to a treaty, purporting to exclude or modify the legal effect of certain provisions in their application to that state. Reservations allow states to join treaties while carving out specific obligations they are unwilling to accept.

The VCLT permits reservations unless they are prohibited by the treaty, the treaty provides that only specified reservations may be made, or the reservation is incompatible with the object and purpose of the treaty. The object and purpose test is the most significant limit: a state cannot make a reservation that guts the treaty's core purpose. For human rights treaties, the human rights treaty bodies have interpreted this test strictly, sometimes invalidating reservations that contracting states intended to be valid.

The United States is a consistent maker of reservations, understandings, and declarations (collectively "RUDs") when joining human rights treaties. When it ratified the International Covenant on Civil and Political Rights in 1992, the US attached RUDs limiting the treaty's domestic application. Other state parties have objected that these RUDs undermine the treaty's object and purpose — a debate with practical consequences for US accountability under international human rights mechanisms.

Pacta Sunt Servanda and Rebus Sic Stantibus

Article 26 of the VCLT codifies the foundational principle of international law: pacta sunt servanda — agreements must be kept. Every treaty in force is binding on the parties and must be performed in good faith. No state may invoke provisions of its domestic law as justification for failure to perform a treaty. The principle is absolute in form, limited in practice by the exceptions that follow it.

The doctrine of rebus sic stantibus — things having thus stood — permits a party to invoke a fundamental change of circumstances as grounds for terminating or withdrawing from a treaty. Article 62 of the VCLT sets strict conditions: the change must not have been foreseen by the parties; the circumstances must have constituted an essential basis of the parties' consent; and the change must radically transform the extent of obligations still to be performed. The doctrine is interpreted narrowly to prevent its use as a pretext for avoiding inconvenient obligations. States rarely succeed in invoking it before international tribunals.

Treaty Interpretation: Three Methodologies

When treaty text is ambiguous, three interpretive approaches compete.

International tribunals formally privilege textual and purposive interpretation but frequently consult travaux préparatoires as a cross-check. The WTO Appellate Body has been criticized for excessive reliance on object and purpose interpretation in a manner that some states argue goes beyond the treaty text — a criticism central to the US blocking of new Appellate Body appointments.

Treaty Withdrawal: The Paris Agreement and INF Treaty

The rules for withdrawing from treaties depend on each treaty's terms. Some treaties expressly permit withdrawal after notice periods; others are silent. When a treaty is silent on withdrawal, the general rule under VCLT Article 56 is that withdrawal is not permitted unless the parties intended to allow it or a right of withdrawal can be implied by the nature of the treaty.

Two high-profile US withdrawals illustrate the legal and political dynamics. The US withdrawal from the Paris Agreement on climate change, announced by President Trump in 2017, was legally straightforward — the agreement permits withdrawal with three years' notice plus one year's processing time. The Biden administration rejoined immediately upon taking office in 2021. The US withdrawal from the Intermediate-Range Nuclear Forces (INF) Treaty in 2019, citing Russian violations, was legally uncomplicated — the treaty expressly permitted six months' notice withdrawal. Russia subsequently withdrew as well, and the treaty ceased to exist.

Jus cogens — peremptory norms of international law that no state may derogate from — represent the ultimate limit on treaty law. A treaty that conflicts with a jus cogens norm is void ab initio (Articles 53 and 64 VCLT). Jus cogens norms include prohibitions on genocide, slavery, torture, and crimes against humanity. States cannot treaty themselves out of these obligations, regardless of what any instrument says.