What Is a Treaty? How Countries Make Promises That Bind Them

The Vienna Convention on the Law of Treaties defines a treaty as an international agreement concluded between states in written form and governed by international law, whatever it is called, so that a convention, a covenant, a protocol, a charter, a pact and an exchange of letters are all treaties if they meet that description. The name signals scale and formality rather than legal effect. What matters is that the parties are states or international bodies with the capacity to make them, that the text is written, and that the parties intended to create legal obligations rather than a political understanding; the Helsinki Final Act of 1975 and most memoranda of understanding are deliberately the second kind, binding in honour and not in law.

The process is a sequence, and the steps matter because a state is not bound until the last of them:

• •Negotiation, by diplomats with full powers from their governments, ending in an agreed text
• •Adoption and authentication of the text, often by signature, which usually binds a state only to refrain from defeating the treaty's purpose
• •Ratification: the domestic step by which the state actually consents, which in most countries requires a parliament and in the United States two thirds of the Senate; a signed but unratified treaty binds nobody, which is why several famous agreements have signatures and no force
• •Entry into force, on the date the treaty sets or when a stated number of states have ratified
• •Reservations: a state may, if the treaty allows, exclude or modify a particular provision for itself, which is how widely different countries can join the same convention
• •Registration with the United Nations, without which a treaty cannot be invoked before its court

The question that occurs to every reader is what happens when a country simply ignores one, and the honest answer is often nothing direct. There is no bailiff. States keep treaties because breaking them costs: retaliation in the same or another area, the loss of reciprocal benefits, exclusion from the arrangements that make trade and travel work, damage to a reputation that every future negotiation depends on, and, in some regimes, actual enforcement, since the body that governs world trade authorises retaliation, the European Court of Human Rights issues judgments that member states comply with in the great majority of cases, and the Security Council can impose sanctions when its permanent members agree. The principle behind all of it is stated in three Latin words in the Vienna Convention, pacta sunt servanda, agreements must be kept, and it is the foundation on which the rest stands.

Treaties provide for their own ending. Most contain a withdrawal clause with a notice period, which is how the United States left the Paris climate agreement and the United Kingdom left the European Union, and the Vienna Convention allows termination by agreement, by a material breach by the other party, by the impossibility of performance, or by a fundamental change of circumstances, which is narrowly construed because every state would like to use it. A treaty may also be superseded by a later one between the same parties, or fall into disuse. What a state cannot do is rely on its own domestic law as an excuse for not performing, a rule that prevents a parliament from voting away an obligation its government accepted.

The reach of treaty law is easy to miss because it works. The Universal Postal Union of 1874 is why a letter posted anywhere arrives anywhere; the Chicago Convention of 1944 built civil aviation; the Geneva Conventions set the rules of war and the treatment of prisoners; the Montreal Protocol of 1987, the most successful environmental treaty ever made, phased out the chemicals destroying the ozone layer and has put the layer on course to recover; the Law of the Sea settled who owns what water; the nuclear non-proliferation treaty has held the number of nuclear states to a fraction of what was predicted in 1960; and the human rights conventions gave individuals, for the first time, claims against their own governments. Each is a promise that a set of sovereign states decided was worth more kept than broken, which is the only mechanism international law has ever had.

A treaty is a written agreement between states intended to create legal obligations, governed by the Vienna Convention of 1969 whatever it is called, made by negotiation, signature, ratification in each country's own system and entry into force, and often modified for particular states by reservations. There is no enforcer, so states keep treaties for reciprocity, reputation and the specific machinery some of them create, and the agreements built on that principle run the post, aviation, the law of war and the recovery of the ozone layer.