Explainer

Secession, decolonisation and state dissolution

Three routes by which new states have emerged, and why the legal questions differ between them.

2 min read · updated 15 September 2026

Decolonisation

In decolonisation, a non-self-governing territory reaches a full measure of self-government through independence, free association or integration.[1] East Timor’s path shows the sequence: under a 1999 agreement, a vote rejecting autonomy led to a UN-led transition, independence in 2002 and UN membership the same year.[2][3]

Secession

Secession is the separation of part of an existing state. Constitutional courts in Canada and Spain have required it to go through constitutional processes: the Supreme Court of Canada held that a province could not secede unilaterally but that a clear expression of will would oblige all parties to negotiate, and Spain’s Constitutional Court declared Catalonia’s 2017 referendum law void.[4][5] Internationally, the ICJ found that Kosovo’s declaration did not breach international law without deciding that a right to secede exists.[6]

Dissolution

Dissolution is the breakup of a state into successor states. In 1991 the Arbitration Committee of the Conference on Yugoslavia found that Yugoslavia was in the process of dissolution and that the republics should settle state succession under international law.[7] It also held that, unless otherwise agreed, former internal boundaries become international frontiers.[8]

Why the categories overlap

Real cases rarely fit one box. A territory can be treated as a colony by the UN while its administrator treats it as an integral province, and a dissolution can begin as a set of secessions. The atlas records the legal context of each case separately rather than assigning one category.

East Timor is an example: the UN never recognised Indonesia’s integration of the territory and kept treating Portugal as the administering power.[9]

Sources for this page

Each number marks a specific claim. Notes give the source, its locator and how it relates to the claim.

  1. 1.

    Claim: Under resolution 1541 (XV), a Non-Self-Governing Territory can be said to have reached a full measure of self-government by emergence as a sovereign independent State, free association with an independent State, or integration with an independent State.

    • United Nations General Assembly, General Assembly resolution 1541 (XV): Principles which should guide Members in determining whether or not an obligation exists to transmit the information called for under Article 73 e of the Charter (United Nations, 15 December 1960) link, annex, principle VI. Source record
  2. 2.

    Claim: Under the 5 May 1999 Agreement, rejection of autonomy in East Timor’s popular consultation would end Indonesia’s links with the territory and start a UN-led transition towards independence.

    • Government of Indonesia; Government of Portugal and Secretary-General of the United Nations, Agreement between the Republic of Indonesia and the Portuguese Republic on the question of East Timor, with the agreements on the modalities for the popular consultation and on security (United Nations, 5 May 1999) link, arts. 5–6, p. 7. Government perspective or primary official source Source record
  3. 3.

    Claim: East Timor became independent on 20 May 2002 and was admitted to the United Nations on 27 September 2002.

    • United Nations Department of Public Information, United Nations Transitional Administration in East Timor (mission page) (United Nations) link, summary. Source record
    • United Nations General Assembly, General Assembly resolution 57/3: Admission of the Democratic Republic of Timor-Leste to membership in the United Nations (United Nations, 27 September 2002) link, operative paragraph. Source record
  4. 4.

    Claim: The Court held that secession of a province could not be achieved unilaterally under the Constitution, but that a clear expression of a desire to secede would create an obligation on all parties to negotiate.

    • Supreme Court of Canada, Reference re Secession of Quebec (Supreme Court of Canada, 20 August 1998) link, paras. 87–88 and 149–151, at pp. 265 and 293. Source record
  5. 5.

    Claim: Spain’s Constitutional Court declared the Catalan referendum law of 2017 unconstitutional and void.

    • Tribunal Constitucional, Sentencia 114/2017, de 17 de octubre de 2017 (Ley 19/2017 del referéndum de autodeterminación) (Agencia Estatal Boletín Oficial del Estado, 24 October 2017) link, Fallo; FJ 2. Source record
  6. 6.

    Claim: In its 2010 advisory opinion the International Court of Justice concluded that Kosovo’s declaration of independence of 17 February 2008 did not violate international law, while stating that it did not need to decide whether international law conferred a positive entitlement to declare independence.

    • International Court of Justice, Accordance with International Law of the Unilateral Declaration of Independence in respect of Kosovo: advisory opinion (case overview) (International Court of Justice) link, case overview. Source record
  7. 7.

    Claim: In Opinion No. 1 (29 November 1991) the Arbitration Committee of the Conference on Yugoslavia found that the Socialist Federal Republic of Yugoslavia was in the process of dissolution and that the republics should settle questions of state succession under international law. Attributed to Arbitration Committee of the Conference on Yugoslavia

    • Pellet, Alain, The Opinions of the Badinter Arbitration Committee: A Second Breath for the Self-Determination of Peoples, European Journal of International Law (1992) link, appendix, Opinion No. 1, p. 183. Source record
  8. 8.

    Claim: In Opinion No. 3 the Committee stated that, except where otherwise agreed, the former internal boundaries become frontiers protected by international law, drawing on the principle of uti possidetis. Attributed to Arbitration Committee of the Conference on Yugoslavia

    • Pellet, Alain, The Opinions of the Badinter Arbitration Committee: A Second Breath for the Self-Determination of Peoples, European Journal of International Law (1992) link, appendix, Opinion No. 3, p. 185. Source record
  9. 9.

    Claim: The United Nations never recognised Indonesia’s 1976 integration of East Timor and continued to regard Portugal as the administering power.

    • United Nations Department of Public Information, East Timor – UNTAET: Background (United Nations, May 2002) link, para. 1. Source record

Source note

Show in the notes list