Abstract
In contemporary times there has been a relaxation of the traditional principle of legality, which implies a restriction of the guarantees of the justiciable in favor of the fight against criminality that offends humanity, which is explained because it is often a criminal manifestation sponsored - or systematically committed - by totalitarian States, which of course would not be interested in legislating criminalizing their own acts. Therefore, in the case of international crimes, legality presupposes the integration of international treaties into domestic legal systems with full effect as prior law to make their sanction viable, even if they were not formally typified in national legislation at the time of their commission.