The Russian war against Ukraine was considered illegal and unjustified and was described as "Aggression" as per the UN General Assembly resolution, while Russia upholds its legitimate right to legitimate self-defense, considering that Ukraine's accession to NATO threatened its security and peace. In the face of the various reactions of states and the United Nations and the consideration by international jurisprudence of several issues related to the Russian-Ukrainian war, the question arises as to the impact of such actions on the interpretation and development of the rules of international law. Therefore, the study focuses - without siding a particular party - on the impact of the Russian-Ukrainian war on the interpretation and development of the rules of international law from two aspects: First: the legitimate use of force, through the analysis of President Putin's speech, in which he presented Russia's justifications for waging war, and its impact on the interpretation and development of the rules of international law related to the legitimate use of force, and exposure to this aspect depends on the analysis of the Russian point of view, which is supported by a few countries. Second: the illegal use of force, and the extent of effectiveness of international legal mechanisms in this respect and how these mechanisms were employed in the conflict and their impact on the interpretation and development of the rules of international law. The exposure to this aspect relies on the analysis of the Ukrainian point of view, which is supported by most states.