The study is devoted to the legal and historiographical interpretation of the inclusion of new territories into Russia in the context of the evolution of domestic constitutional regulation and international norms. The aim is to identify the logic behind the formation of the national mechanism for changing the composition of the federation and to assess its correlation with the dominant approaches of international law and academic doctrine. The sources include the Constitution of the Russian Federation, federal constitutional laws on the admission of new constituent entities, decisions of the Constitutional Court, key instruments of universal and regional international law, as well as Russian and foreign scholarly literature on the issues of sovereignty, territorial integrity, self-determination, and border changes. The methodology comprises formal legal, comparative legal, and historical legal methods, analysis of law enforcement practice, and a historiographical review of key approaches. It is demonstrated that in the early 2000s a detailed constitutional and legal mechanism for admitting new entities was established, based on the conclusion of an international treaty, its review by the constitutional supervisory body, and the subsequent adoption of a special federal constitutional law, which was adapted to the changing political context during the events of 2014 and 2022. Judicial practice shows a shift in argumentation from an emphasis on the right of peoples to self-determination and the historical connection of territories toward a strengthening of humanitarian rhetoric and the motive of protecting the population, while maintaining the priority of the domestic constitutional order over international obligations. Comparison with the positions of international organizations and leading schools of international law reveals a persistent conflict between national and international legal systems, within which the inclusion of disputed territories is interpreted as a completed constitutionally formalized accession within Russia and as an unlawful annexation in most foreign and international assessments. Special attention is paid to the practical consequences for the regulation of citizenship, the operation of legislation, recognition of acts of public authorities, and the legal status of documents in the incorporated territories, as well as to the reflection of these processes in contemporary scholarly literature of Russia and foreign countries. The study substantiates the conclusion on the formation of a state-centered model of legal understanding, in which national sovereignty and the autonomy of the constitutional order are placed above universalist interpretations of international norms, creating a long-term regime of legal uncertainty regarding the status of the new regions and necessitating further interdisciplinary research into the consequences of such divergence.