In the shadow of geopolitical tensions, the ongoing conflict in Ukraine stands as a pivotal event in global affairs. This book delves into the heart of the matter through the lens of international law and in particular of jus ad bellum. The book begins with an in-depth historical analysis. It then examines key doctrines such as the right to self-determination, remedial secession, collective self-defense, and intervention by invitation, scrutinizing them with scholarly precision. Drawing parallels to past global conflicts and legal precedents, the author dissects the arguments invoked by Russia, exploring the echoes of doctrines previously employed by other major powers.