Treaty, in Public International Law, is an agreement or contract between two or more separate states, by means of which the jural relations subsisting between them are defined. As treaties are merely contracts, the rules for their negotiation, from a jural point of view, do not differ from those regulating the constitution of valid contracts in any other department of jurisprudence. Thus, in order to the validity of a treaty, the parties to it must be capable of contracting—a condition which renders void contracts entered into by protected states or members of a confederation when such contracts are in excess of the powers retained by these states; the agents employed must be duly empowered to contract on behalf of their respective states; the situation of the parties must be such that their consent may be regarded as having been freely given; and the objects of the agreement must be, physically and morally, possible and generally in conformity with law. No special form is prescribed by international usage as necessary for treaties. From the moment that consent on both sides is clearly established an obligatory treaty exists; but generally of course international contracts are, as a matter of prudence, consigned to writing. Sometimes treaties take the form of a specific agreement signed by both parties; but more frequently they take the form of a joint declaration or of an exchange of diplomatic notes. In practice the word treaty is commonly used for the larger political or commercial contracts, the term convention being applied to agreements on points of minor importance. When, as is usually the case, a treaty is entered into through the agency of official plenipotentiaries, it is necessary to its validity that it be ratified, either expressly or tacitly, by the supreme treaty-making power of the state. The constitution of each particular state determines in whom resides this supreme power of contracting treaties with foreign powers. In monarchies, whether absolute or constitutional, it is usually vested in the sovereign. In Great Britain the exercise of this power is subject to parliamentary censure; and ministers who advise the conclusion of any treaty which shall afterwards be judged derogatory to the honour or disadvantageous to the welfare of the nation are liable to impeachment, a proceeding of which English history affords numerous instances. In republics the chief magistrate, senate, or executive council is usually entrusted with the sovereign power. By the constitution of the United States the president prepares treaties; he discusses the conditions with foreign governments, and signs the treaty; but, over and above this, a sitting of the senate is required to approve the treaty, and no less than a majority of two-thirds in order to ratify it.
Concerning treaties, considered as sources of international law, there is a wide difference of opinion. Many Continental publicists regard treaties, or a certain class of them, as forming a sort of international jurisprudence; while on the other hand most British and American writers are disposed to lay small stress on them. Treaties assented to by all or nearly all civilised states, and avowedly making changes in the law, or laying down new rules, are important in proportion to the number of states which sign them. If they are accepted by all civilised states, or if, without this formal acceptance, the rules contained in them are tacitly adopted by many states, the treaties undoubtedly are sources of international law. Such treaties are rare, the Declaration of Paris in 1856 and the Geneva Convention of 1864 being the most conspicuous examples. Another class of treaties which form an important source of international law are those which are expressly declaratory of that law as understood by the con- tracting parties: e.g. the Protocol signed at the Conference of London in 1871 and the Three Rules of the Treaty of Washington in the same year. On the other hand, treaties signed by two or three states only, and stipulating for a new rule or rules as between the contracting parties, are evidence of what international law is not rather than of what it is. If, however, the new rule works well, and is gradually adopted by other states, it may win its way into the international code. Far the most numerous class of treaties are those merely settling some matter of dispute between the parties to them; these contain no rules of international conduct, and do not in any way affect international law.
Performance of the stipulations contained in a treaty used frequently to be secured by the taking of hostages. This is now in disuse, the treaty of Aix-la-Chapelle in 1748 being the last occasion upon which hostages were given, except in military conventions. The occupation of territory and the mortgaging of revenue have also often been used as modes of taking security. The most common means of securing the execution of a treaty is now through a treaty of guarantee by a third power. Treaties are interpreted according to practically the same rules as the contracts of private individuals. Collections of treaties are either general or national. The former class are made up of selections from the treaties of all nations; the latter include only the treaties to which a particular country was a party. The chief national collection of British treaties is that of L. Hertslet, librarian of the Foreign Office, continued by his son and successor, Sir Edward Hertslet.