State succession in international law refers to the replacement of one state by another in the responsibility for the international relations of a territory. This can occur through various events such as decolonization, unification, dissolution, secession, or transfer of territory. The legal implications of such changes are complex, giving rise to different theories and affecting a range of rights and duties.

Theories of State Succession:

1. Universal Succession (Old View): This traditional theory, largely discredited today, posited that the successor state automatically inherits all the rights and obligations of the predecessor state. It treated the state as a legal entity that simply changes its governing body, with all its legal personality, including treaties, debts, and property, passing seamlessly to the new entity. This theory proved impractical and incompatible with the concept of state sovereignty, especially in cases of decolonization.

2. Clean Slate (Nihilist or Tabula Rasa) Theory (Modern View): This is the dominant theory, particularly for newly independent states emerging from colonial rule. It holds that the successor state begins with a 'clean slate,' meaning it is not automatically bound by the treaties, debts, or other obligations of the predecessor state. The new state has the option to choose which treaties and obligations it wishes to continue. This theory emphasizes the sovereignty of the new state to determine its own international relations. However, there are important exceptions, especially concerning 'dispositive' or 'territorial' treaties.

3. Continuity Theory: This theory applies when one state absorbs another, or when there is a merger of states. In such cases, the legal personality of the absorbing or continuing state is considered to persist, and it assumes the rights and obligations of the absorbed state. For example, when West Germany absorbed East Germany, the Federal Republic of Germany continued its legal personality and assumed the obligations of the former East Germany.

Rights and Duties Arising Out of State Succession:

State succession impacts various aspects of international law, leading to specific rights and duties for the successor state:

1. Treaties:

   • Boundary Treaties: These are generally considered to 'run with the land' (uti possidetis juris principle) and are binding on the successor state, regardless of the clean slate principle. This ensures stability of international borders.
   • Human Rights Treaties: There is a growing trend, supported by UN human rights bodies, towards automatic succession to human rights treaties due to their universal character and the protection they offer to individuals.
   • Other Treaties (Political, Economic, etc.): For most other treaties, the clean slate principle often applies, especially for newly independent states. However, multilateral conventions may allow for notification of succession, enabling the new state to declare its adherence.

2. Public Property: Generally, public property (land, buildings, assets) located in the territory that is subject to succession passes to the successor state. This includes state archives, which are crucial for the administration and historical record of the territory.

3. Public Debts: This is a highly complex area. The 1983 Vienna Convention on Succession of States in Respect of State Property, Archives and Debts (though not widely ratified) suggests that public debts should pass to the successor state in an equitable proportion. However, in practice, this is often subject to negotiation between the predecessor and successor states, taking into account factors like the origin of the debt and the benefits derived from it.

4. Nationality: The successor state has the right to determine the nationality of the inhabitants of the territory. Typically, individuals habitually resident in the territory at the time of succession are granted the nationality of the successor state, often with options for dual nationality or choice in certain circumstances.

5. Membership in International Organizations: Membership in international organizations (like the UN) is generally not automatic. The successor state usually needs to apply for new membership, although there can be exceptions or special arrangements (e.g., in cases of unification or dissolution where one entity continues).

The legal framework for state succession is primarily governed by customary international law, supplemented by the 1978 Vienna Convention on Succession of States in Respect of Treaties and the 1983 Vienna Convention on Succession of States in Respect of State Property, Archives and Debts, though neither convention has achieved widespread ratification, indicating the persistent complexities and political sensitivities surrounding the issue.