The repression of war crimes, crimes against humanity, and genocide, whatever the nationality of the offender and the place where they are committed, is crucial to ensuring respect for international law and to the interests of justice. The chief responsibility for this repression lies with States. The substantive and procedural criminal law and the judicial system of each State must enable it to prosecute and bring to trial persons allegedly responsible for these crimes. States must also be able to offer the assistance required from them when procedures to that end are undertaken abroad or by an international jurisdiction. International law, especially in view of the very nature of these crimes, lays down certain conditions that prosecution and sentencing by national courts must meet. To the extent that these are respected, States are free to decide their own rules in this matter.