Criminal law and proceedings are at the heart of State sovereignty, and cooperation in criminal matters is a voluntary undertaking; a State is not obliged to cooperate with others in criminal matters unless it has agreed to do so. But, over time, the parochial view that criminal law, including its effects, is local in nature has given way to an ever-growing need for and actual regulation of international legal cooperation. Influential factors in this regard are increased cross-border activities, including the commission of crimes, international terrorism and the development of human rights.
International crimes are of concern to all States and therefore lend themselves to efforts at cooperation. A commitment to cooperate, in the form of extradition, is the alternative to prosecution in accordance with the aut dedere aut judicare principle, when applicable. Cooperation is particularly important when the State is exercising jurisdiction over crimes committed abroad, but may also be necessary when a State is investigating and prosecuting crimes committed on its own territory. Prosecution of genocide, crimes against humanity and war crimes is no exception.
But international law, treaty and custom, has not (yet) developed a special regime for State-to-State cooperation concerning these international crimes. The Geneva Conventions and Additional Protocol I, for example, explicitly refer to cooperation in accordance with domestic legislation. A recent attempt in the UN Commission on Crime Prevention and Criminal Justice to create a special cooperation instrument for such crimes, in furtherance of the ICC complementarity principle (Chapter 8), was rejected on formal grounds, but clearly also the substance was considered controversial. One must therefore resort to general principles and provisions of international and domestic law on international cooperation in criminal matters. In relation to the ICTY, ICTR and ICC, however, State cooperation is subject to separate regimes to which we shall return in Chapter 20.