The open access publication of this book has been published with the support of the Swiss National Science Foundation. International law is increasing in relevance to the topic of secession. This book demonstrates that if a secessionist entity’s effectiveness is achieved in violation of peremptory norms, the emergence of statehood is precluded, thereby challenging a classical view of secession as purely factual and meta-legal. Dr. Júlia Miklasová coins the term “illegal secessionist entity,” demonstrates the pervasive effects of the original illegality on the subsequent relations of such entities (purported diplomatic, treaty, economic relations, acts and laws) and outlines the overlapping regimes of the law of occupation, human rights law and duty of non-recognition. Post-Soviet secessionist entities result from an illegal use of force. They are thus prohibited from becoming States, and further consequences of their illegality apply.