JOSEF MRÁZEK CYIL 7 ȍ2016Ȏ the relationship between the UN Charter provisions of Art. 2 (4) and customary international law on the use of force, with the problems of unilateral military use of force, “authorization” of military actions by the UNSC, the definition of aggression and the concept of self-defence. It was argued in the past by some prominent lawyers that Art. 2 (4) would allow unilateral use of military force, if not directed against territorial integrity, or political independence or is not inconsistent in any other manner with the purposes of the UN Charter. Many other theories or doctrines, supporting the practice of some states, to some extent have already eroded international law on the use of force. There are various ideas on the use of force, acknowledging that there might be situations when imperative political and moral considerations dictate the states to act “outside the law”. The Report of Iraq Inquiry (Sir Chillcot report) may be cited as an example of the abuse of political decisions. Some lawyers insist that in international law the Latin proverb “ ex inuiria ius orbitur ” is valid. A very substantial question is what differences exists between customary prohibition on the use of force and prohibition in the UN Charter or how international customs have so far “modified” Art. 2 (4). Another important issue is the relationship between the “legality” and “legitimacy” of armed actions. It seems, in the light of political developments in today’s world, that prohibition on the use of force is losing its legal and political strength and meaning. The author deals critically with the definitions of “aggression”, armed attack and self-defence, with the rescue of nationals abroad, with intervention on invitation, self-defence and armed attack by non-state actors or with cyber operations relating to the use of force. The article stresses the relevance of the UN Charter and is in favour of rigid interpretation of Arts. 2 (4) and 51. The interplay of treaty law and international customary law exists without any doubt. Art. 31 (3) (b) of the Vienna Convention on the Law of Treaties recognizes state practice as a primary element of treaty interpretation. The wording of the prohibition of the use of force in the UNSC Charter leads to some divergent interpretations, often influenced by the political, military and economic interests of the relevant states. The attempt of the UNGA Special Committee on Enhancing the Effectiveness of the Principle of Non-Use of Force in 1981 to work out the World Treaty on the Non-Use of Force was not successful owing to the divergent opinions of the states. 80 Already in 1970 T.M. Franck had asked a question of “who killed Art. 2 (4) of the UN Charter?” His reply was that the renunciation of the use of force by the individual state is only reasonable when it is assured that the UNSC is able to take steps for the prohibition to succeed. Otherwise, in his view, Art. 2 (4) has become void. 81 After 80 Doc. A/36/41, GAOR, Supp. No. 41. 81 FRANCK ,T. M. Who killed Art. 2 (4): or: The Changing Norms Governing the Use of Force by States. AJIL . Vol. 64, 1970, n. 2, pp. 809-837.