This book concerns the relationship between two broad elements underpinning international law: legality and effectiveness. The element of legality is embodied in the maxim ex injuria jus non oritur, according to which a violation of international law cannot produce legally valid outcomes. In some situations, however, effective practice does generate normative consequences, particularly through international human rights law. This phenomenon is embodied in the maxim ex factis jus oritur. This book studies the balance between the two elements as it is expressed in a discrete category of situations: the transition from a territorial regime that is illegal under international law to a legal territorial regime.

The term ‘territorial regime’ denotes an entity in which a functioning governing apparatus exercises control over a population in a territory, and makes a claim of sovereignty over that territory. Territorial status is a matter not only of fact but also of law. A territorial regime is considered in this book ‘illegal’ when its creation involves the violation of a peremptory norm of international law, principally the prohibition on the use of force or the obligation to respect the right of peoples to self-determination. This is not to say that territorial claims do not at times involve other violations of international law, but those are less likely to result in an objective illegality of the regime.