A treaty is a binding agreement under international law concluded by subjects of international law, namely states and international organizations. Treaties can be called by many names: treaties, international agreements, protocols, covenants, conventions, exchanges of letters, exchanges of notes, etc.; however all of these are equally treaties, and the rules are the same regardless of what the treaty is called.

Treaties can be loosely compared to contracts: both are means of willing parties assuming obligations among themselves, and a party to either that fails to live up to their obligations can be held legally liable for that breach. The central principle of treaty law is expressed in the maxim pacta sunt servanda--"pacts must be respected."

Vienna Convention

The 1969 Vienna Convention on the Law of Treaties is the authoritative treaty on the international law of treaties, establishing the procedures by which treaties are adopted, interpreted, and invalidated. It is considered mostly a codification of already existing and binding customary law on treaties, and so aside from some necessary gap-filling and clarification, it is not viewed as a change in existing international law. This means that unlike most treaties, the Vienna Convention could arguably be binding to even non-parties. There is a Vienna Convention on the Law of Treaties between States, and on the Law of Treaties between States and International Organizations or between International Organizations. Most states have ratified these treaties. The Vienna Convention on Succession of States with Respect to Treaties has comparatively few parties. Many states reject its provisions as not adequately reflecting the customary international law on the subject.

Consequences of terminology

One significant part of treaty making is that signing a treaty implies recognition that the other side is a sovereign state and that the agreement being considered is enforceable under international law. Hence, nations can be very careful about terming an agreement to be a treaty. For example, within the United States agreements between states are compacts and agreements between states and the federal government or between agencies of the government are memoranda of understanding. Conversely, perhaps the most significant thing about the Anglo-Irish Treaty was that it was explicitly a treaty and hence implied British recognition of Irish sovereignty.

Another situation can occur when one party wishes to create an obligation under international law, but the other party does not. Such as the case with the Sino-British Joint Declaration signed between the United Kingdom and the People's Republic of China in 1984. The wording for the document was very carefully designed to make it possible to interpret it either as a treaty or as a communique. Similar factors have been at work with respect to discussions between North Korea and the United States over security guarantees and nuclear proliferation.

The terminology can also be confusing because a treaty may and usually is named something other than a treaty, such as a convention, protocol, or simply agreement. Conversely some legal documents such as the Treaty of Waitangi are internationally considered to be documents under domestic law.

Bilateral and multilateral treaties

A multilateral treaty has several parties, and establishes rights and obligations between each party and every other party. Multilateral treaties are often, but not always, open to any state; others are regional.

Bilateral treaties by contrast are negotiated between a limited number of states, most commonly only two, establishing legal rights and obligations between those two states only. It is possible however for a bilateral treaty to have more than two parties; consider for instance the bilateral treaties between Switzerland and the European Union (EU) following the Swiss rejection of the European Economic Area agreement. Each of these treaties has seventeen parties. These however are still bilateral, not multilateral, treaties. The parties are divided into two groups, the Swiss ("on the one part") and the EU and its member states ("on the other part"). The treaty establishes rights and obligations between the Swiss and the EU and the member states severally; it does not establish any rights and obligations amongst the EU and its member states.

Reservations

Reservations are essentially caveats to a state's acceptance of a treaty. These can be qualifications or denials of specific obligations, or the insistence upon particular interpretations of treaty language. These must be included at the time of signing or ratification--a party cannot add a reservation after it has already joined a treaty.

Some treaties expressly forbid all reservations or just specific ones. Treaties may also expressly authorize certain reservations. Otherwise, reservations may be permitted to the extent that they are not inconsistent with the goal and purpose of the treaty. Though the allowance of reservations means that not all parties to a treaty will be bound under the same precise obligations, states will often nevertheless consent to reservations to gain as many parties to the treaty as possible for the sake of a general goal.

If a reservation is considered unacceptable, the reserving party may be excluded from the treaty, or the reservation may be considered severable from the party's ratification, meaning that the reservation is ignored but the party is still bound under the treaty.

Execution and implementation

Treaties may be seen as 'self-executing', in that merely becoming a party puts the treaty and all of its obligations in action. Other treaties may be non-self-executing and require 'implementing legislation'--a change in the domestic law of a state party that will direct or enable it to fulfill treaty obligations. An example of a treaty requiring such legislation would be one mandating local prosecution by a party for particular crimes.

The division between the two is often not clear and is often politicized in disagreements within a government over a treaty, as a non-self-executing treaty cannot be acted upon without the proper change in domestic law. If a treaty requires implementing legislation, a state may be in default of its obligations by the failure of its legislature to pass the necessary domestic laws.

Interpretation

The language of treaties, like that of any law or contract, must be interpreted when the wording does not seem clear or it is not immediately apparent how it should be applied in a perhaps unforeseen circumstance. The Vienna Convention states that treaties are to be interpreted “in good faith” according to the “ordinary meaning given to the terms of the treaty in their context and in the light of its object and purpose.” International legal experts also often invoke the 'principle of maximum effectiveness,' which interprets treaty language as having the fullest force and effect possible to establish obligations between the parties.

No one party to a treaty can impose its particular interpretation of the treaty upon the other parties. Consent may be implied, however, if the other parties fail to explicitly disavow that initially unilateral interpretation, particularly if that state has acted upon its view of the treaty without complaint. Consent by all parties to the treaty to a particular interpretation has the legal effect of adding an additional clause to the treaty--this is commonly called an 'authentic interpretation.'

International tribunals and arbitors are often called upon to resolve substantial disputes over treaty interpretations. To establish the meaning in context, these judicial bodies may review the preparatory work from the negotiation and drafting of the treaty as well as the final, signed treaty itself.

Invalidation

Articles 46-53 of the Vienna Convention set out the only ways that treaties can be invalidated--considered unenforceable and void under international law. A treaty will be invalidated due to either the circumstances by which a state party joined the treaty, or due to the content of the treaty itself. Invalidation is separate from withdrawal, suspension, or termination (addressed below), which all involve an alteration in the consent of the parties of a previously valid treaty rather than the invalidation of that consent in the first place.

Ultra vires treaties

A party's consent to a treaty is invalid if it was given by an agent or body without power to do so under that state's domestic law. States are reluctant to inquire into the internal affairs and processes of other states, and so a “manifest” violation is required such that it would be “objectively evident to any State dealing with the matter". A strong presumption exists internationally that a head of state has acted within his proper authority. It seems that no treaty has ever actually been invalidated on this provision.

Consent is also invalid if it is given by a representative who ignored restrictions he is subject to by his sovereign during the negotiations, if the other parties to the treaty were notified of those restrictions prior to his signing.

Misunderstanding, fraud, corruption, coercion

A state's consent may be invalidated if there was an erroneous understanding of a fact or situation at the time of conclusion, which formed the "essential basis" of the state's consent. Consent will not be invalidated if the misunderstanding was due to the state's own conduct, or if the truth should have been evident.

Consent will also be invalidated if it was induced by the fraudulent conduct of another party, or by the direct or indirect "corruption" of its representative by another party to the treaty. Coercion of either a representative, or the state itself through the threat or use of force, if used to obtain the consent of that state to a treaty, will invalidate that consent.

Peremptory norms

A treaty is null and void if it is in violation of a peremptory norm. These norms, unlike other principles of customary law, are recognized as permitting no violations and so cannot be altered through treaty obligations. These are limited to such universally accepted prohibitions as those against genocide, slavery, torture, and piracy, meaning that no state can legally assume an obligation to commit or permit such acts.

Ending treaty obligations

Withdrawal

Treaties are not necessarily permanently binding upon the signatory parties. As obligations in international law are traditionally viewed as arising only from the consent of states, many treaties expressly allow a state to withdraw as long as it follows certain procedures of notification. Many treaties expressly forbid withdrawal. Other treaties are silent on the issue, and so if a state attempts withdrawal through its own unilateral denunciation of the treaty, a determination must be made regarding whether permitting withdrawal is contrary to the original intent of the parties or to the nature of the treaty. Human rights treaties, for example, are generally interpreted to exclude the possibility of withdrawal, because of the importance and permanence of the obligations. Withdrawal by a party may also be permitted if all other parties under the treaty consent.

If a state party's withdrawal is successful, its obligations under that treaty are considered terminated, and withdrawal by one party from a bilateral treaty of course terminates the treaty. When a state withdraws from a multi-lateral treaty, that treaty will still otherwise remain in force between the other parties.

Suspension and termination

If a party has materially violated, or breached, its treaty obligations, the other parties may invoke this breach as grounds for temporarily suspending their obligations to that party under the treaty. A material breach may also be invoked as grounds for permanently terminating the treaty itself.

A treaty breach does not automatically suspend or terminate treaty relations, however. The issue must be presented to an international tribunal or arbitror (usually specified in the treaty itself) to legally establish that a sufficiently serious breach has in fact occurred. Otherwise, a party that prematurely and perhaps wrongfully suspends or terminates its own obligations due to an alleged breach itself runs the risk of being held liable for breach. Additionally, parties may choose to overlook treaty breaches while still maintaining their own obligations towards the party in breach.

Treaties sometimes include provisions for self-termination, meaning that the treaty is automatically terminated if certain defined conditions are met. Some treaties are intended by the parties to be only temporarily binding and are set to expire on a given date. Other treaties may terminate if a defined event occurs if the treaty is meant to exist only under certain conditions or in the absence thereof.

A party may claim that a treaty should be terminated, even absent an express provision, if there has been a fundamental change in circumstances. Such a change is sufficient if it was unforeseen, if it undermined the “essential basis” of consent by a party, if it radically transforms the extent of obligations between the parties, and if the obligations are still to be performed. A party cannot base this claim on change brought about by its own breach of the treaty. This claim also cannot be used to invalidate treaties that established or redrew political boundaries.

Treaties and indigenous peoples

Treaties formed an important part of European colonization and, in many parts of the world, Europeans attempted to legitimize their sovereignty by signing treaties with indigenous peoples. In most cases these treaties were in extremely disadvantageous terms to the native people, who often did not appreciate the implications of what they were signing.

In some rare cases, such as with Ethiopia and Qing Dynasty China, the local governments were able to use the treaties to at least mitigate the impact of European colonization. This involved learning the intricacies of European diplomatic customs and then using the treaties to prevent a power from overstepping their agreement or by playing different powers against each other.

In other cases, such as New Zealand and Canada, treaties allowed native peoples to maintain a minimum amount of autonomy. Such treaties between colonizers and indigenous peoples are an important part of political discourse in the late 20th and early 21st century, but the treaties being discussed are internationally considered to be part of the nation's domestic law, and to have little international standing.

Role of the United Nations

The United Nations Charter states that treaties must be registered with the UN to be invoked before it or enforced in its judiciary organ, the International Court of Justice. This was done to prevent the proliferation of secret treaties that occurred in the 19th and 20th century. The Charter also states that its members' obligations under it outweigh any competing obligations under other treaties.

United States law

Article II, Section 2 of the United States Constitution grants power to the President to make treaties with the "advice and consent" of two thirds of the Senate. This is difference from normal legislation which requires approval by simple majorities in both the Senate and the House of Representatives.

However, throughout U.S. history, the President has also made "international agreements" through congressional-executive agreements (CEAs) that are ratified with only a majority from both houses of Congress, or sole executive agreements made by the President alone. Though the constitution does not expressly provide for any alternative procedure and although some noted constitutional scholars, such as Laurence Tribe, believe that CEAs are unconstitutional, the Supreme Court has considered these agreements to be valid, and that any disagreements are a political question for the executive and legislative branches to work out amongst themselves. In addition, U.S. law distinguishes between self-executing treaties, which do not require additional legislative action, and non-self-executing treaties which do require the enactment of new laws.

These distinctions of procedure and terminology do not affect the binding status of such agreements under international law. Nevertheless, they do have major implications under U.S. domestic law. In Missouri v. Holland, the Supreme Court ruled that the power to make treaties under the U.S. Constitution is a power separate from the enumerated powers of the federal government, and hence the federal government can use treaties to legislate in areas which would otherwise fall within the exclusive competence of the states. By contrast, a congressional-executive agreement can only cover matters which the Constitution explicitly places within the powers of Congress and the President, while an executive agreement can only cover matters within the President's authority or matters in which Congress has delegated authority to the President.

While the ratification process for treaties is different from the process for CEA's, which venue is more advantageous for passage depends on the relevant circumstances. In general, arms control agreements are ratified by the treaty mechanism because it is simpler to go through one house of congress than two. At the same time, trade agreements are generally voted on as a CEA because the two-thirds requirement makes it possible for agricultural interests to veto any tariff reduction in the Senate.

The United States takes a different view concerning the relationship between international and domestic law than many other nations, particularly in Europe. Unlike nations which view international agreements as always superseding national law, the American view is that international agreements become part of the body of U.S. federal law. As a result, Congress can modify or repeal treaties by subsequent legislative action, even if this amounts to a violation of the treaty under international law. The most recent changes will be enforced by U.S. courts entirely independently of whether the international community still considers the old treaty obligations binding upon the U.S. Additionally, an international agreement that is inconsistent with the U.S. Constitution is void under domestic U.S. law, the same as any other federal law in conflict with the Constitution, and the Supreme Court could rule a treaty provision to be unconstitutional and void under domestic law, although it has never done so. The constitutional constraints are stronger in the case of CEA and executive agreements, which cannot override the laws of state governments.

The Supreme Court has also ruled in Goldwater v. Carter, 444 U.S. 996 (1979) that the President has the power to unilaterally abrogate a treaty without the consent of Congress or the Senate. The case in question involved President Jimmy Carter's termination of a defense treaty with the Republic of China on Taiwan.

The U.S. is not a party to the Vienna Convention. However, the State Department has nonetheless taken the position that it is still binding, in that the Convention represents established customary law. The U.S. habitually includes in treaty negotiations the reservation that it will assume no obligations that are in violation of the U.S. Constitution.