At its first session, in 1949, the Commission selected the law of treaties as a topic for codification to which it gave priority. The Commission appointed J. L. Brierly, Sir Hersch Lauterpacht, Sir Gerald Fitzmaurice and Sir Humphrey Waldock as the successive Special Rapporteurs for the topic at its first, fourth, seventh and thirteenth sessions, in 1949, 1952, 1955 and 1961, respectively. The Commission considered the topic at its second, third, eighth, eleventh and thirteenth to eighteenth sessions, in 1950, 1951, 1956, 1959 and from 1961 to 1966, respectively. In connection with its work on the topic, the Commission had before it the reports of the Special Rapporteurs,[1] information provided by Governments [2]  as well as documents prepared by the Secretariat. [3]

Extracts from The Work of the International Law Commission, 6th ed., vol. 1

The Commission had originally envisaged its work on the law of treaties as taking the form of “a code of a general character”, rather than of one or more international conventions. In its report on its eleventh session, in 1959, to the General Assembly, the Commission stated:

“In short, the law of treaties is not itself dependent on treaty, but is part of general customary international law. Queries might arise if the law of treaties were embodied in a multilateral convention, but some States did not become parties to the convention, or became parties to it and then subsequently denounced it; for they would in fact be or remain bound by the provisions of the treaty in so far as these embodied customary international law de lege lata. No doubt this difficulty arises whenever a convention embodies rules of customary international law. In practice, this often does not matter. In the case of the law of treaties it might matter — for the law of treaties is itself the basis of the force and effect of all treaties. It follows from all this that if it were ever decided to cast the Code, or any part of it, in the form of an international convention, considerable drafting changes, and possibly the omission of some material, would almost certainly be required.” [4]

At its thirteenth session, in 1961, the Commission changed the scheme of its work from a mere expository statement of the law of treaties to the preparation of draft articles capable of serving as a basis for an international convention. This decision was explained as follows by the Commission in its report on its fourteenth session, in 1962:

“First, an expository code, however well formulated, cannot in the nature of things be so effective as a convention for consolidating the law; and the consolidation of the law of treaties is of particular importance at the present time when so many new States have recently become members of the international community. Secondly, the codification of the law of treaties through a multilateral convention would give all the new States the opportunity to participate directly in the formulation of the law if they so wished; and their participation in the work of codification appears to the Commission to be extremely desirable in order that the law of treaties may be placed upon the widest and most secure foundations.” [5]

The General Assembly, in resolution 1765 (XVII) of 20 November 1962 (E, F, S, R, C, A), recommended that the Commission continue the work on the law of treaties, taking into account the views expressed in the Assembly and the written comments submitted by Governments.

At its fourteenth to sixteenth sessions, from 1962 to 1964, the Commission proceeded with the first reading of the draft articles and submitted the provisionally adopted draft articles to Governments for comment. The Commission completed the first reading of the draft articles at its sixteenth session, in 1964.

At its seventeenth session, in 1965, the Commission began the second reading of the draft articles in the light of the comments of Governments. It re-examined the question of the form ultimately to be given to the draft articles, and adhered to the views it had expressed in 1961 and 1962 in favour of a convention. The Commission noted that, at the General Assembly’s seventeenth session, in 1962, the Sixth Committee had stated in its report that the great majority of representatives had approved the Commission’s decision to give the codification of the law of treaties the form of a convention.

At its eighteenth session, in 1966, the Commission completed the second reading of the draft articles and adopted its final report on the law of treaties, setting forth seventy-five draft articles together with their commentaries. [6]  In submitting the final report to the General Assembly, the Commission recommended that the Assembly should convene an international conference of plenipotentiaries to study the Commission’s draft articles on the law of treaties and to conclude a convention on the subject. [7]

In drawing up the draft articles, the Commission decided to limit the scope of application of those articles to treaties concluded between States, to the exclusion of treaties between States and other subjects of international law (e.g., international organizations) and between such other subjects. It also decided not to deal with international agreements not in written form. In addition, the Commission decided that the draft articles should not contain any provisions concerning the following topics: the effect of the outbreak of hostilities upon treaties; succession of States in respect of treaties; the question of the international responsibility of a State with respect to a failure to perform a treaty obligation; “most-favoured-nation clause”; and the application of treaties providing for obligations or rights to be performed or enjoyed by individuals. [8]

Following the discussion in the Sixth Committee on the report of the Commission on the work of its eighteenth session, the General Assembly by resolution 2166 (XXI) of 5 December 1966 (E, F, S, R, C, A) decided to convene an international conference of plenipotentiaries to consider the law of treaties and to embody the results of its work in an international convention and such other instruments as it may deem appropriate. It requested the Secretary-General to convoke the first session of the conference early in 1968 and the second session early in 1969. By the same resolution, the Assembly invited MemberStates, the Secretary-General and the Directors-General of those specialized agencies which act as depositaries of treaties to submit their written comments and observations on the draft articles. The International Atomic Energy Agency also submitted written comments and observations.

The following year, on the recommendation of the Sixth Committee, the General Assembly, by resolution 2287 (XXII) of 6 December 1967 (E, F, S, R, C, A), decided to convene the first session of the United Nations Conference on the Law of Treaties at Vienna in March 1968.

The first session of the United Nations Conference on the Law of Treaties was accordingly held at Viennafrom 26 March to 24 May 1968 and was attended by representatives of 103 countries and observers from thirteen specialized and intergovernmental agencies. The second session was held from 9 April to 22 May 1969, also at Vienna, and was attended by representatives of 110 countries and observers from fourteen specialized and intergovernmental agencies. [9]  The first session of the Conference was devoted primarily to consideration by a Committee of the Whole and by a Drafting Committee of the set of draft articles adopted by the International Law Commission. The first part of the second session was devoted to meetings of the Committee of the Whole and of the Drafting Committee, completing their consideration of articles reserved from the previous session. The remainder of the second session was devoted to thirty plenary meetings which considered the articles adopted by the Committee of the Whole and reviewed by the Drafting Committee.

The Conference adopted the Vienna Convention on the Law of Treaties [10]  on 22 May 1969. The Convention is made up of a preamble, eighty-five articles and an annex.

In line with the draft articles prepared by the Commission, the Vienna Convention on the Law of Treaties applies to treaties between States, the term “treaty” being defined for the purposes of the Convention as “an international agreement concluded between States in written form and governed by international law, whether embodied in a single instrument or in two or more related instruments and whatever its particular designation”. Without prejudice to any relevant rules of the organization concerned, the Convention expressly provides that it applies to any treaty which is the constituent instrument of an international organization and to any treaty adopted within an international organization. Part I of the Convention also provides that the fact that international agreements concluded between States and other subjects of international law or between such other subjects of international law, or international agreements not in written form, are not covered by the Convention shall not affect (a) the legal force of such agreements, (b) the application to them of any of the rules set forth in the Convention to which they would be subject under international law independently of the Convention, and (c) the application of the Convention to the relations of States as between themselves under international agreements to which other subjects of international law are also parties. Finally, it is also provided that the Convention applies only to treaties which are concluded by States after the entry into force of the Convention with regard to such States, without prejudice to the application of any of the rules set forth in the Convention to which treaties would be subject under international law independently of the Convention.

The principal matters covered in the Convention are: conclusion and entry into force of treaties (part II), including reservations and provisional application of treaties; observance, application and interpretation of treaties (part III), including treaties and third States; amendment and modification of treaties (part IV); invalidity, termination and suspension of the operation of treaties (part V), including the procedure for the application of the provisions of that part and for the settlement of disputes concerning the application or interpretation of those provisions, and the consequences of the invalidity, termination or suspension of the operation of a treaty; miscellaneous provisions (part VI), reserving cases of State succession, State responsibility and outbreak of hostilities, as well as the case of an aggressor State, and dealing with the severance or absence of diplomatic or consular relations and the conclusion of treaties; and depositaries, notifications, corrections and registration (part VII). The conciliation procedure referred to in article 66 of part V is specified in an annex to the Convention. The text of the Convention is reproduced in annex V, section F.

The final provisions of the Convention open it for signature and for ratification or accession by all States Members of the United Nations or members of any of the specialized agencies or of the International Atomic Energy Agency or parties to the Statute of the International Court of Justice, and also by any other State invited by the General Assembly to become a party to the Convention. The Convention was opened for signature on 23 May 1969. It remained open for signature until 30 November 1969at the Federal Ministry for Foreign Affairs of Austria and, subsequently, until 30 April 1970, at United Nations Headquarters. Signatures are subject to ratification. The Convention is open for accession by any non-signatory State entitled to become a party. It entered into force on 27 January 1980. By 20 October 2003, ninety-six States were parties to the Convention.

In addition to the Vienna Convention on the Law of Treaties, the Conference adopted two declarations (the Declaration on the Prohibition of Military, Political or Economic Coercion in the Conclusion of Treaties and the Declaration on Universal Participation in the Vienna Convention on the Law of Treaties) and five resolutions which were annexed to the Final Act of the Conference. [11]

In the Declaration on Universal Participation in the Vienna Convention on the Law of Treaties, the Conference stated its conviction that multilateral treaties which deal with the codification and progressive development of international law, or the object and purpose of which are of interest to the international community as a whole, should be open to universal participation; noted that articles 81 and 83 of the Vienna Convention on the Law of Treaties enable the General Assembly to issue special invitations to States which are not members of the United Nations or of any of the specialized agencies or of the International Atomic Energy Agency, or parties to the Statute of the International Court of Justice, to become parties to the Convention; and invited the General Assembly to give consideration, at its twenty-fourth session, to the matter of issuing invitations in order to ensure the widest possible participation in the Vienna Convention on the Law of Treaties. At the General Assembly’s twenty-fourth session, this matter was referred to the Sixth Committee, which recommended to the Assembly that the question of issuing invitations be deferred until the twenty-fifth session. The Assembly adopted this recommendation without objection. On the recommendation of the General Committee, the General Assembly further deferred the consideration of the matter in 1970, 1971, 1972 and 1973 until the following year. On 12 November 1974, the Assembly adopted resolution 3233 (XXIX) (E, F, S, R, C, A) whereby it decided to invite all States to become parties to the Vienna Convention on the Law of Treaties.