The ‘Kursk Operation’ marks a pivotal moment in the War in Ukraine, shifting the dynamics of this conflict and raising some legal questions regarding the Ukrainian incursion into Russian Territory. This paper will examine the context that prompts what might be the third phase of the Ukrainian counteroffensive, examining the legal arguments that make this act lawful and compliant with Article 51 of the UN Charter, which grants States the right to self-defence. By analysing the right to self-defence, alongside the principles of proportionality and necessity, this paper argues that the Kursk Operation can be lawfully considered an act of self-defence. Lastly, it examines the definition of "occupation" to clarify the legal distinctions relevant to this operation.