The invasion of Ukraine by Russia in 2022 constituted an egregious violation of one of the fundamental principles of international law: the prohibition of the use of force against the territorial integrity or political independence of any State, enshrined in Article 2(4) of the UN Charter. This violation capacitates Ukraine to rightfully invoke the right of collective self-defense under Article 51 of the UN Charter. This Article finds that some of the criteria to invoke and exercise this right have been fulfilled, and the rest can also be fulfilled – allowing other countries to lawfully engage in Ukraine’s collective self-defense. Additionally, the Article argues that despite claiming to aid in Ukraine’s individual self-defense, through the significant military aid and logistical support amounting to the use of force provided to Ukraine, the assisting States are already indirectly practicing the right of collective self-defense, albeit without formally invoking it.