A treaty is a formal written agreement entered into by actors in international law, namely sovereign states and international organizations.^[1] A treaty may also be known as an international agreement, protocol, covenant, convention, pact, or exchange of letters, among other terms. Regardless of terminology, all these instruments may be considered treaties subject to the same rules under international law.^[2]

Treaties can be loosely compared to contracts, in that the parties willingly assume binding obligations among themselves, and any party that breaches its obligations can be held liable under international law.^[3] Treaties vary significantly in substance and complexity, and may govern a wide variety of issues, such as affirming political boundaries, facilitating trade and commerce, or ending a military conflict.

International law on treaties has largely been codified by the Vienna Convention on the Law of Treaties (VCLT), which sets forth the rules and procedures for making, enforcing, and interpreting treaties.^[4] As one of the earliest manifestations of international relations, treaties have long been recognized as a primary source of international law.^[5]

A treaty is an official, express written agreement that states use to legally bind themselves.^[6] A treaty is the official document which expresses that agreement in words; it is also the objective outcome of a ceremonial occasion which acknowledges the parties and their defined relationships. There is no prerequisite of academic accreditation or cross-professional contextual knowledge required to publish a treaty.

Since the late 19th century, most treaties have followed a fairly consistent format. A treaty typically begins with a preamble describing the "High Contracting Parties" and their shared objectives in executing the treaty, as well as summarizing any underlying events (such as the aftermath of a war in the case of a peace treaty). Modern preambles are sometimes structured as a single very long sentence formatted into multiple paragraphs for readability, in which each of the paragraphs begins with a gerund (desiring, recognizing, having, etc.).

The High Contracting Parties; referred to as either the official title of the head of state (but not including the personal name), e.g. His Majesty The King of X or His Excellency The President of Y, or alternatively in the form of "Government of Z"; are enumerated, and along with the full names and titles of their plenipotentiary representatives, and a boilerplate clause about how their representatives have communicated (or exchanged) their full powers (i.e., the official documents appointing them to act on behalf of their respective high contracting party) and found them in good or proper form. However, under the Vienna Convention on the Law of Treaties if the representative is the head of state, head of government or minister of foreign affairs, no special document is needed, as holding such high office is sufficient.

The end of the preamble and the start of the actual agreement is often signaled by the words "have agreed as follows".

After the preamble comes numbered articles, which contain the substance of the parties' actual agreement. Each article heading usually encompasses a paragraph. A long treaty may further group articles under chapter headings.

Modern treaties, regardless of subject matter, usually contain articles governing where the final authentic copies of the treaty will be deposited and how any subsequent disputes as to their interpretation will be peacefully resolved.

The end of a treaty, the eschatocol (or closing protocol), is often signaled by a clause like "in witness whereof" or "in faith whereof", the parties have affixed their signatures, followed by the words "DONE at", then the site(s) of the treaty's execution and the date(s) of its execution. The date is typically written in its most formal, non-numerical form. For example, the Charter of the United Nations was "DONE at the city of San Francisco the twenty-sixth day of June, one thousand nine hundred and forty-five". If the treaty is executed in multiple copies in different languages, that fact is always noted, and is followed by a stipulation that the versions in different languages are equally authentic.

The signatures of the parties' representatives follow at the very end. When the text of a treaty is later reprinted, such as in a collection of treaties currently in effect, an editor will often append the dates on which the respective parties ratified the treaty and on which it came into effect for each party.

Bilateral treaties are concluded between two states or entities.^[7] It is possible for a bilateral treaty to have more than two parties; for example, each of the bilateral treaties between Switzerland and the European Union (EU) has seventeen parties: 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.

A multilateral treaty is concluded among several countries, establishing rights and obligations between each party and every other party. ^[7] Multilateral treaties may be regional or may involve states across the world.^[8] Treaties of "mutual guarantee" are international compacts, e.g., the Treaty of Locarno which guarantees each signatory against attack from another.^[7]

Reservations are essentially caveats to a state's acceptance of a treaty. Reservations are unilateral statements purporting to exclude or to modify the legal obligation and its effects on the reserving state.^[9] These must be included at the time of signing or ratification, i.e. "a party cannot add a reservation after it has already joined a treaty". Article 19 of Vienna Convention on the law of Treaties in 1969.

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.^[10]

There are three ways an existing treaty can be amended. First, formal amendment requires State parties to the treaty to go through the ratification process all over again. The re-negotiation of treaty provisions can be long and protracted, and often some parties to the original treaty will not become parties to the amended treaty. When determining the legal obligations of states, one party to the original treaty and one a party to the amended treaty, the states will only be bound by the terms they both agreed upon. Treaties can also be amended informally by the treaty executive council when the changes are only procedural, technical change in customary international law can also amend a treaty, where state behavior evinces a new interpretation of the legal obligations under the treaty. Minor corrections to a treaty may be adopted by a procès-verbal; but a procès-verbal is generally reserved for changes to rectify obvious errors in the text adopted, i.e. where the text adopted does not correctly reflect the intention of the parties adopting it.

In international law and international relations, a protocol is generally a treaty or international agreement that supplements a previous treaty or international agreement. A protocol can amend the previous treaty, or add additional provisions. Parties to the earlier agreement are not required to adopt the protocol. Sometimes this is made clearer by calling it an "optional protocol", especially where many parties to the first agreement do not support the protocol.

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.

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, since a non-self-executing treaty cannot be acted on 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.

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 another 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.

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.

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 definition of the English word 'Treaty' varies depending on professional context(s).

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. For example, the Single Convention on Narcotic Drugs provides that the treaty will terminate if, as a result of denunciations, the number of parties falls below 40. Many treaties expressly forbid withdrawal. Article 56 of the Vienna Convention on the Law of Treaties provides that where a treaty is silent over whether or not it can be denounced there is a rebuttable presumption that it cannot be unilaterally denounced unless:

The possibility of withdrawal depends on the terms of the treaty and its *travaux preparatoire.*It has, for example, been held that it is not possible to withdraw from the International Covenant on Civil and Political Rights. When North Korea declared its intention to do this the Secretary-General of the United Nations, acting as registrar, said that original signatories of the ICCPR had not overlooked the possibility of explicitly providing for withdrawal, but rather had deliberately intended not to provide for it. Consequently, withdrawal was not possible.^[11]

In practice, because of sovereignty, any state can purport to withdraw from any treaty at any time, and cease to abide by its terms. The question of whether this is lawful can be regarded as the success or failure to anticipate community acquiescence or enforcement, that is, how other states will react; for instance, another state might impose sanctions or go to war over a treaty violation.

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 terminates the treaty. When a state withdraws from a multilateral treaty, that treaty will still otherwise remain in force among the other parties, unless, it otherwise should or could be interpreted as agreed upon between the remaining states parties to the treaty.

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.^[12]

A treaty breach does not automatically suspend or terminate treaty relations, however. It depends on how the other parties regard the breach and how they resolve to respond to it. Sometimes treaties will provide for the seriousness of a breach to be determined by a tribunal or other independent arbiter.^[13] An advantage of such an arbiter is that it prevents a party from prematurely and perhaps wrongfully suspending or terminating its own obligations due to another's alleged material 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.^[14]

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.^[14]

There are several reasons an otherwise valid and agreed upon treaty may be rejected as a binding international agreement, most of which involve problems created at the formation of the treaty. For example, the serial Japan-Korea treaties of 1905, 1907 and 1910 were protested;^[15] and they were confirmed as "already null and void" in the 1965 Treaty on Basic Relations between Japan and the Republic of Korea.^[16]

A party's consent to a treaty is invalid if it had been given by an agent or body without power to do so under that state's domestic laws. 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.

According to the preamble in The Law of Treaties, treaties are a source of international law. If an act or lack thereof is condemned under international law, the act will not assume international legality even if approved by internal law.^[17] This means that in case of a conflict with domestic law, international law will always prevail.^[18]

Articles 46–53 of the Vienna Convention on the Law of Treaties 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 above), 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.

A governmental leader'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.

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 the aggressive use of force, genocide and other crimes against humanity, piracy, hostilities directed at civilian population, racial discrimination and apartheid, slavery and torture,^[19] meaning that no state can legally assume an obligation to commit or permit such acts.^[20]

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. Section 103 of the Charter also states that its members' obligations under it outweigh any competing obligations under other treaties.

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^[21], giving a comparison between modern treaty law and the historical Articles of Confederation.

The constitution of Australia allows the executive government to enter into treaties, but the practice is for treaties to be tabled in both houses of parliament at least 15 days before signing. Treaties are considered a source of Australian law but sometimes require an act of parliament to be passed depending on their nature. Treaties are administered and maintained by the Department of Foreign Affairs and Trade, which advised that the "general position under Australian law is that treaties which Australia has joined, apart from those terminating a state of war, are not directly and automatically incorporated into Australian law. Signature and ratification do not, of themselves, make treaties operate domestically. In the absence of legislation, treaties cannot impose obligations on individuals nor create rights in domestic law. Nevertheless, international law, including treaty law, is a legitimate and important influence on the development of the common law and may be used in the interpretation of statutes."^[22] Treaties can be implemented by executive action, and often, existing laws are sufficient to ensure a treaty is honoured.

Australian treaties generally fall under the following categories: extradition, postal agreements and money orders, trade and international conventions.

The federal constitution of Brazil states that the power to enter into treaties is vested in the president of Brazil and that such treaties must be approved by the Congress of Brazil (Articles 84, Clause VIII, and 49, Clause I). In practice, that has been interpreted as meaning that the executive branch is free to negotiate and sign a treaty but that its ratification by the president requires the prior approval of Congress. Additionally, the Supreme Federal Court has ruled that after ratification and entry into force, a treaty must be incorporated into domestic law by means of a presidential decree published in the federal register for it to be valid in Brazil and applicable by the Brazilian authorities.

The court has established that treaties are subject to constitutional review and enjoy the same hierarchical position as ordinary legislation (leis ordinárias, or "ordinary laws", in Portuguese). A more recent ruling by the Supreme Court of Brazil in 2008 has altered that somewhat by stating that treaties containing human rights provisions enjoy a status above that of ordinary legislation, subject to only the constitution itself. Additionally, the 45th Amendment to the constitution makes human rights treaties approved by Congress by a special procedure enjoy the same hierarchical position as a constitutional amendment. The hierarchical position of treaties in relation to domestic legislation is of relevance to the discussion on whether and how the latter can abrogate the former and vice versa.

The constitution does not have a supremacy clause with the same effects as the one in the US constitution, which is of interest to the discussion on the relation between treaties and legislation of the states of Brazil.

In India, subjects are divided into three lists: union, state and concurrent. In the normal legislation process, the subjects on the union list must be legislated by the Parliament of India. For subjects on the state list, only the respective state legislature can legislate. For subjects on the concurrent list, both governments can make laws. However, to implement international treaties, Parliament can legislate on any subject and even override the general division of subject lists.

In the United States, the term "treaty" has a different, more restricted legal sense than in international law. US law distinguishes what it calls "treaties" from "executive agreements", which are either "congressional-executive agreements" or "sole executive agreements". The classes are all equally treaties under international law; they are distinct only in internal US law.

The distinctions are primarily concerning their method of approval. Treaties require advice and consent by two thirds of the Senators present, but sole executive agreements may be executed by the President acting alone. Some treaties grant the President the authority to fill in the gaps with executive agreements, rather than additional treaties or protocols. Finally, congressional-executive agreements require majority approval by both the House and the Senate before or after the treaty is signed by the President.

Currently, international agreements are ten times more likely to be executed by executive agreement. Despite the relative ease of executive agreements, the President still often chooses to pursue the formal treaty process over an executive agreement to gain congressional support on matters that require the Congress to pass implementing legislation or appropriate funds as well as for agreements that impose long-term, complex legal obligations on the US. For example, the deal by the United States, Iran, and other countries is not a treaty.

See the article on the Bricker Amendment for history of the relationship between treaty powers and Constitutional provisions.

The US Supreme Court ruled in the Head Money Cases that "treaties" do not have a privileged position over Acts of Congress and can be repealed or modified, for the purposes of US law, by any subsequent Act of Congress, just like any other regular law. The court also ruled in Reid v. Covert that treaty provisions that conflict with the US Constitution are null and void under US law.

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. 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, under the doctrine of terra nullius (later overturned by Mabo v Queensland, establishing the concept of native title well after colonization was already a fait accompli). 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.

Prior to 1871, the government of the United States regularly entered into treaties with Native Americans but the Indian Appropriations Act of March 3, 1871 (ch. 120, 16 Stat. 563) had a rider (25 U.S.C. § 71 ^[42] ) attached that effectively ended the President’s treaty making by providing that no Indian nation or tribe shall be acknowledged as an independent nation, tribe, or power with whom the United States may contract by treaty. The federal government continued to provide similar contractual relations with the Indian tribes after 1871 by agreements, statutes, and executive orders.^[23]