Efforts made since the early twentieth century to stop forcible territorial acquisition through international law have been at the centre of the international community’s pursuit of peace and security based on international law. Until 2022, post-war efforts based on the United Nations (UN) Charter seemed to have succeeded in preventing attempts by nuclear-weapon States to forcibly acquire another State’s internationally recognised territory. Regrettably, this situation turned out to be a temporary success, and it is the right time to review the efforts made so far to stop forcible territorial acquisition and to examine the current state of the rules of international law. This book is based on the approach often taken by practitioners, who increasingly rely on views expressed by courts and tribunals and search for narrative based on evidence and logic which work for their legal diplomacy. As the scope of the prohibition of acquisition of territory by force (‘PATBF’) and its relationship with the prohibition of use of force (‘PUF’) have not yet been clarified, this book will try to give the best possible views based on available evidence. Where PATBF is difficult to apply, PUF may work as an alternative rule against potential attempts to acquire territory by force.