This essay addresses the issue regarding the law making by the International Court of Justice in its contentious function. This problem is addressed from the analysis of the case concerning military and paramilitary activities in and against Nicaragua. In particular, the essay analyses the determination of the effective control standard for the attribution of responsibility for acts of a person or group of persons, and the relevance of the so-called secondary rules in international law. Through a doctrinal and jurisprudential study, the author demonstrates the important role of secondary rules in international law, and therefore, the implications on domestic law of the law of the fact that secondary rules are created principally by international tribunals.