The definition of 1933 may offer valuable characteristics for establishing the aggressor, but one does not get around the fact that a formal legal definition shows the impossibility of doing justice to all actual political cases.
With the attempt to set down a new regulation for creating order in the world in the Charter of the United Nations, one returned, having obviously recognized this truth, to the idea of a decision by an international organ without wanting to force its judgment into the inconvenient form of a rigid definition. The Charter of Peace of San Francisco says, in Chapter VII, Article 39:
“The Security Council shall determine the existence of any threat to world peace and security or breach of the peace or act of aggression and shall make recommendations or decide what measures shall be taken to maintain or restore international peace and security.”
In the year 1939 there was neither a recognized definition of the term aggressor nor an institution authorized to designate the aggressor.
The League of Nations as an instrument for the settlement of disputes had completely failed. This was expressed outwardly already by the fact that three great powers had left it. How little the mutilated League of Nations was taken notice of in international life, was shown by the attitude of the Soviet Union in the Finnish question. She did not take into consideration in any way the decision of the League of Nations with regard to this conflict but pursued her own interests in her dealings with Finland.
If now, after these statements, I make a proposal to the Court as to what should be understood by the concept of attack in Article 6(a) of the Charter, this qualification cannot be related to a definition recognized in international law. There is nothing left but adherence to the interpretation which the practice of states and the traditions of diplomacy are wont to give.
According to the conception prevailing in the year 1939, the outbreak of war, in whatever way it happened, was not legally appraised. The Kellogg Pact and the negotiations following it have not been able to abolish this fact, which was a result of centuries of development. This is deeply to be regretted, but one cannot ignore reality. The fact that this opinion, when war broke out, is in accordance with the conception of international law of the main participating powers that had signed the Charter, follows from the fact that men of international reputation in the field of international law were of the opinion that, should the Kellogg Pact and the system of collective security fail, the traditional legal conception as to war was still valid.[[D]]
[D] Oppenheim-Lauterpacht, International Law. 5th Edition. Page 154.