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  • Print publication year: 2018
  • Online publication date: April 2019

3 - Law of treaties



The rapid development of international law in the post–World War II period from 1945 onwards also coincided with greater reliance upon treaties as a source of international law. Treaties had always been recognised as a source of international law, and their status was confirmed in art 38 of the Statute of the International Court of Justice (ICJ Statute). However, it was the development of the United Nations (UN), with its emphasis upon international law and focus upon treaty-making as a means of not only peaceful resolution of disputes but also multilateral agreement between States on matters of common concern, that was the catalyst for a greater reliance upon treaties in the conduct of international relations.

This chapter begins by briefly examining the growth of treaty-making, before considering what a treaty is, including the nature of ‘instruments of less than treaty status’. It then covers treaty negotiation, creation and entry into force; reservations, objections and declarations to treaties; and legal obligations arising once a treaty has entered into force. The chapter then turns to treaty interpretation, and then examines how treaties are voided or ended through invalidity, suspension or termination. Finally, the chapter concludes with a discussion of amendment and modification of treaties.

Growth of treaty-making

The UN has actively promoted reliance upon treaties as a means of developing international law, with the International Law Commission (ILC) tasked in its early years with the responsibility of drafting a number of treaties, which were then presented for negotiation and consideration at a multilateral conference of UN member States. Through this process, international law rapidly developed in the postwar period of the late 1940s to 1960s in areas such as international humanitarian law, the law of the sea, and human rights. While these developments were occurring at the multilateral level under the sponsorship of either the UN or other developing international organisations, there was also a growth in bilateral treaty–making as States began developing their postwar economies and rebuilding their international relations across many different fields of interest. This phenomenon in international treaty-making was also gradually duplicated at the regional level as groups of States with shared interests came together to reach settlement upon matters of common interest.

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Aust, Anthony, Modern Treaty Law and Practice, Cambridge University Press, Cambridge, 2013
Corten, Oliver and Klein, Pierre (eds), The Vienna Conventions on the Law of Treaties: A Commentary, Oxford University Press, Oxford, 2011
Gardiner, Richard, Treaty Interpretation, Oxford University Press, Oxford, 2008
Greig, D. W., Invalidity and the Law of Treaties, British Institute of International and Comparative Law, London, 2006
Sloss, David (ed), The Role of Domestic Courts in Treaty Enforcement, Cambridge University Press, New York, 2009