The creation of States has often been compared to a meta-judicial fact which cannot be explained by legal rules. International law does not permit secession, but does not prohibit it either. Thus, the only criterion for the emergence of a new State, outside the colonial context, is the principle of effectiveness: if a secessionist entity succeeds in fulfilling the conditions of statehood, a new State is born. Secession is not a question of law, but a question of fact. The goal of this chapter is to examine this traditional view and to discover the exact interactions between the law and the facts in the process of the creation of States. Part I of the chapter confirms the actuality of the principle of effectiveness. Of course, no real ‘automatism’ exists in this field. As the study shows, in some cases the effectiveness of the secession did not permit the creation of a new State. In other cases, a State has been created without fulfilling the conditions for statehood. In a third type of case, the principle of uti possidetis juris clashed with the principle of effectiveness. Nonetheless, it remains the case that appeal to the principle of effectiveness is the only available solution in a society marked by a great paradox: States, which in the great majority prohibit unilateral secession in their internal legal orders, do not wish (or do not think it feasible) to create a prohibitive rule in the international legal order.