States regularly proclaim the sanctity of treaty obligations and few principles are as firmly established as pacta sunt servanda. Yet, treaty breaches are by no means exceptional: adapting one of international law's most celebrated statements, one might even say that 'almost all nations, almost all the time, consider their rights under a given treaty to be violated: By way of a snapshot, at the time of writing, eleven of fourteen active contentious cases pending before the International Court of Justice (ICJ) involve claims, by one State, that a certain treaty has been violated. And this ignores the many treaty breaches that do not reach the spotlight, but are addressed quietly ( eg by means of a phone call between representatives of the States concerned) or are not addressed at all. Against this background, a recent textbook is surely right to state that 'most disputes submitted to international adjudication involve some problem of treaty interpretation.

The frequency of real or alleged treaty breaches is neither a source for major concern, nor should it come as a great surprise. While some argue that States today are more cautious before entering into new treaties, especially multilateral ones, the last 150 years have seen a significant trend towards 'treatificatioii, and treaty commitments have come to cover ever greater areas of international relations. That some of these commitments should occasionally be breached is only natural: nobody is perfect, and States certainly are not. More mportantly, not all treaty breaches are intentional, or show disrespect for international law as a system, let alone its ground rule of pacta sunt servanda. Often, breaches result from mere oversights or lack of information: to give just one example, before States like Paraguay, Germany, and Mexico were beginning to raise the matter, few people were likely to be aware of the requirements imposed by the Vienna Convention on Consular Relations (VCCR) with respect to consular notification. Or, treaty disputes may be due to different, plausible interpretations of a given treaty commitment-for example, with respect to the scope of an obligation to prevent the commission of acts of genocide, as was the case in the proceedings between Bosnia and Serbia before the ICJ. Finally, at times, conflicting obligations may even require States to disregard obligations arising under one treaty to comply with the demands of another-in which case, conflict resolution techniques such as the lex specialis principle, or jus cogens, may clarify questions of precedence. And, of course, not every treaty breach is in itself dramatic; the spectrum of what is covered by the term 'treaty breach' is huge. It comprises acts of aggression amounting to a large-scale violation of Article 2( 4) of the UN Charter just as it does one State's imposition of an 11 per cent ad valorem import tax on foreign goods where a treaty binds the tariff to 10.9 per cent.

Against this background, it seems natural that real or alleged treaty breaches are by no means an exceptional feature of international law, shaped by well over a century of treatification. The real question is whether international law provides means and methods to respond to them. This chapter addresses that question. We do so in four steps. First, we provide an overview of the international regime governing reactions against treaty breaches. In the next two sections, we analyse the two most relevant generally available means of response under the law of treaties and the law of State responsibility respectively. Our final section offers some concluding observations. In addressing questions of treaty breaches and responses, we will focus on rules of international law regulating inter-State behaviour. Notwithstanding this restriction, it seems clear that treaty breaches can be committed by and against different (non-State) subjects of international law, notably by and against international organizations. While these raise some special problems ( eg relating to determining whether the organization itself or its members bear responsibility), they are in principle subject to the rules developed to govern inter-State relations.