The article traces the main lessons of Russian-Ukrainian war as of 2025. Recognizing the conflict in its continuity since 2014 as transformative for contemporary international relations, it offers a structured periodization and names the important stages. Five key lessons are identified: the return of industrial-scale warfare, the importance of strategic communication, fragmentation of the existing global order, the crisis in strategic imagination as well as the risks of conflict spillover. The study concludes that international relations are in the era of great-power competition with no clear vision of a sustainable security order.