According to the positivist conception, the State is the legal entity formed by the combination of three components: a population, a territory and an effective government meaning a government that really has an effective monopoly of coercion over its territory and its population. The reconstruction of the defaulting State involves the re-uniting of these three elements. But what is lacking in the State after a crisis is the existence of an effective government. And the absence of an effective government is a threat both to the security of the population and to international security. The object of this paper is to study State reconstruction in cases where a State previously existed but also to look at where the European Union helped to build new States like for instance Kosovo and other Balkan States