(b) The agents acting for the state must be duly authorized, i.e. the plenipotentiaries must act within their powers.
(c) There must be freedom of consent in the agreements between the states. This does not imply that force, as by war, reprisals, or otherwise, may not be used in bringing about a condition of affairs which may lead a state, without parting with its independence, to make such sacrifices as may be necessary to put an end thereto. No constraint can be put upon the negotiators of the treaty by threats of personal violence, or in any way to prohibit their free action, without invalidating their acts. There is no freedom of consent when the agreement is reached through fraud of either party, and treaties so obtained are not valid.
(d) The treaties must be in conformity to law, as embodied in the generally recognized principles of international law and the established usage of states. States could not by treaty appropriate the open sea, protect the slave trade, partition other states unless as a measure of self-protection, deprive subjects of essential rights of humanity, or enter into other agreements that could not be internationally obligatory.
[§ 85. Classification of Treaties]
Treaties have been variously classified, but the classifications serve no great purpose. The most common classification is clearly set forth by Calvo. As regards form, treaties may be, (1) transitory, or (2) permanent or perpetual; as regards nature, (1) personal, relating to the sovereign, or (2) real, relating to things and not dependent on the sovereign person; as regards effects, (1) equal or (2) unequal, or according to other effects, simple or conditional, definitive or preliminary, principal or accessory, etc.; as regards objects, (1) general or (2) special.[277] In a narrower sense treaties may be divided into many classes, as political, economic, guarantee, surety, neutrality, alliance, friendship, boundary, cession, exchange, jurisdiction, extradition, commerce, navigation, peace, etc., and conventions relating to property of various kinds, including literary and artistic, to post and telegraph, etc. Most of these classes are sufficiently described by their titles. The nature of some of the classes is not fully indicated in the title.
A treaty of guarantee is an engagement by which a state agrees to secure another in the possession of certain specified rights, as in the exercise of a certain form of government, in the free exercise of authority within its dominions, in freedom from attack, in the free navigation of specified rivers, in the exercise of neutrality, etc. In 1831 and 1839, by the Treaties of London, the independence and neutrality of Belgium were guaranteed, and in the Treaty of 1832 the affairs in Greece were adjusted under guarantee. The Treaty of Paris, 1856, guarantees "the independence and the integrity of the Ottoman Empire." When the guaranteeing state is not only bound to use its best efforts to secure the fulfillment of the treaty stipulations, but to make good the conditions agreed upon in the treaty provided one of the principals fails to meet its obligations, the treaty is not merely one of guarantee, but also a treaty of surety. This happens in case of loans more particularly.
Agreements of states to act together for specific or general objects constitute treaties of alliance. The nature of these treaties of alliance varies with the terms. They may be defensive, offensive, equal, unequal, general, special, permanent, temporary, etc., or may combine several of these characteristics.
[§ 86. Interpretation of Treaties]
Sometimes clauses interpreting treaties are discussed and adopted by the states signing a treaty. These acts may take the form of notes, protocols, declarations, etc. The dispatch of the French ambassador at London, Aug. 9, 1870, to the foreign secretary interprets certain clauses of the treaty guaranteeing the neutrality of Belgium. In cases where no preliminary agreement in regard to interpretation is made, there are certain general principles of interpretation which are ordinarily accepted. Many treatises follow closely the chapters of Grotius and Vattel upon this subject.[278]
The rules usually accepted are: (1) Words of the treaty are to be taken in the ordinary and reasonable sense as when elsewhere used under similar conditions. (2) If the words have different meanings in the different states, the treaty should so far as possible be construed so as to accord with the meaning of the words in the states which accepted the conditions. (3) In default of a plain meaning, the spirit of the treaty or a reasonable meaning should prevail. (4) Unless the fundamental rights of states are expressly the subject of the agreement, these rights are not involved. (5) That which is clearly granted by the treaty carries with it what is necessary for its realization.