A Treaty is an agreement under international law entered into by actors in international law, namely states and international organizations. A Treaty may also be known as: (international) agreement, protocol, covenant, convention, exchange of letters, exchange of notes, accord, memorandum of understanding, etc. Regardless of the terminology, all of these international agreements under international law are equally treaties and the rules are the same. Note that in United States constitutional law, the term "treaty" has a special meaning which is more restricted than its meaning in international law; see below.
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 liable under international law for that breach. The central principle of treaty law is expressed in the maxim pacta sunt servanda—"pacts must be respected".
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.
Originally, international law was unaccepting of treaty reservations, rejecting them unless all parties to the treaty accepted the same reservations. However, in the interest of encouraging the largest number of states to join treaties, a more permissive rule regarding reservations has emerged. While some treaties still expressly forbid any reservations, they are now generally permitted to the extent that they are not inconsistent with the goals and purposes of the treaty.
When a state limits its treaty obligations through reservations, other states party to that treaty have the option to accept those reservations, object to them, or object and oppose them. If the state accepts them (or fails to act at all), both the reserving state and the accepting state are relieved of the reserved legal obligation as concerns their legal obligations to each other (accepting the reservation does not change the accepting state's legal obligations as concerns other parties to the treaty). If the state opposes, the parts of the treaty affected by the reservation drop out completely and no longer create any legal obligations on the reserving and accepting state, again only as concerns each other. Finally, if the state objects and opposes, there are no legal obligations under that treaty between those two state parties whatsoever. The objecting and opposing state essentially refuses to acknowledge the reserving state is a party to the treaty at all.
Some examples: the United Nations Framework Convention on Climate Change (UNFCCC) established a framework for the development of binding greenhouse gas emission limits, while the Kyoto Protocol contained the specific provisions and regulations later agreed upon.
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.
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 arbiters 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.
Another situation can occur when one party wishes to create an obligation under international law, but the other party does not. This factor has 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.
A treaty breach does not automatically suspend or terminate treaty relations, however. The issue must be presented to an international tribunal or arbiter (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 self-terminate if the treaty is meant to exist only under certain conditions.
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 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.
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.
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.
After their adoption, treaties as well as their amendments have to follow the official legal procedures of the United Nations, as applied by the Office of Legal Affairs, including signature, ratification and entry into force.
In function and effectiveness, the UN has been compared to the pre-Constitutional United States Federal government by some, giving a comparison between modern treaty law and the historical Articles of Confederation.
Currently, international agreements are executed by executive agreement rather than treaties at a rate of 10:1. Despite the relative ease of executive agreements, the President still often chooses to pursue the formal treaty process over an executive agreement in order to gain Congressional support on matters that require the Congress to pass implementing legislation or appropriate funds, and those agreements that impose long-term, complex legal obligations on the U.S.
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. In the case of indigenous Australians, unlike with the Māori of New Zealand, no treaty was ever entered into with the indigenous peoples entitling the Europeans to land ownership. Such treaties between colonizers and indigenous peoples are an important part of political discourse in the late 20th and early 21st century, the treaties being discussed have international standing as has been stated in a treaty study by the UN.