The legal act is a manifestation of Will expressed in order to produce legal effects. In public international law the basic legal act is the international treaty. The treaty represents the legal act that expresses, in writing, the agreement of will intervened between subjects of international law in order to create, modify or extinguish rights and obligations in the relations between them and is governed by international law (art.2(1) of the 1969 Vienna Convention on the law of treaties). States are the most important actors in the international arena. Frontiers define limits of the state territory, i.e. territory on which the sovereignty of this or that state extends. Adoption of laws and their enforcement, tax collection, foreign trade activities, and others within the given state are carried out by the State. State sovereignty is recognized by other states through the establishment of diplomatic relations and, as a rule, through the membership in the United Nations.