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A political union is a type of state which is composed of or created out of smaller states. The process is called unification and repeat unifications are referred to as reunifications. Unlike a personal union, the individual states share a central government and the union is recognized internationally as a single political entity. A political union may also be called a legislative union or state union.
A union may be effected in a number of forms, broadly categorized as:
- Incorporating union
- Incorporating annexation
- Federal (or confederal) union
- Federative annexation
- Mixed unions.
Incorporating union 
In an incorporating union a new state is created, the former states being entirely dissolved into the new state (albeit that some aspects may be preserved; see below "Preservation of interests").
Examples of incorporating union 
- South Africa (1910)
- Spain (process from 1037 to 1479) — however the realms of the Crown of Aragon were formally separate from the Crown of Castile were not administratively unified with Castile until 1716 and 1833 respectively.
- Yemen (1990)
Preservation of interests 
Nevertheless a full incorporating union may preserve the laws and institutions of the former states, as happened in the creating of the United Kingdom. This may be simply a matter of practice or to comply with a guarantee given in the terms of the union. For example:
- In the annexation of Brittany to France in 1488, a guarantee was given as to the continuance of laws and of the Estates of Brittany (a guarantee revoked in 1789 at the French Revolution).
- The Treaty of Union for creating the united Kingdom of Great Britain in 1707 contained a guarantee of the continuance of the civil laws and the existing courts in Scotland (a continuing guarantee).
- In the Union creating the United Kingdom of Great Britain and Ireland in 1801, no such guarantee was given for the laws and courts of Ireland, though they were continued as a matter of practice.
- Tyrol was guaranteed that its Freischütz companies would not be posted to fight outside Tyrol without their consent (a guarantee revoked by the Austrian republic).
Incorporating annexation 
In an incorporating annexation a state or states is united to and dissolved in an existing state, whose legal existence continues.
Annexation may be voluntary or, which is more frequent, by conquest .
Examples of incorporating annexation 
- England formally annexed Wales under the two Laws in Wales Acts of 1535 and 1542
- Prussia/Germany (1864, 1866 and 1870–71)
- Italian unification (1860–1861)
- Serbia (with Montenegro) (1918)
- Serbia and Montenegro (2003–2006)
Federal or confederal union 
In a federal or confederal union the states continue in existence but place themselves under a new federal authority. The federal state alone will be the state in international law though the federated states retain an existence in domestic law.
Examples of federal or confederal union 
- Australia (1901)
- Cameroon (1961–1970)
- Canada (1867)
- German Empire (1871–1919)
- India (1950)
- Polish–Lithuanian union (1569–1791)
- Serbia and Montenegro (2003–2006)
- Switzerland (confederation from 1291)
- Tanzania (1964)
- The United Arab Emirates (1971)
- Union of Soviet Socialist Republics (1922–1991)
- The United States of America (in confederal union under the Articles of Confederation, beginning in 1781 and becoming a federal union under the United States Constitution in 1788)
Federal or confederal annexation 
If a state becomes a federated unit of another existing state, the former continuing its legal existence, then that is a federal annexation. The new federated state thus ceases to be a state in international law but retains its legal existence in domestic law, subsidiary to the federal authority.
Examples of federal annexation 
- British Columbia with Canada (1871)
- Eritrea with Ethiopia (1951 to 1962)
- Geneva with Switzerland (1815)
- Newfoundland with Canada (1949)
- Prince Edward Island with Canada (1873)
- Texas with the United States of America (1846)
Mixed unions 
The unification of Italy involved a mixture of unions. The kingdom consolidated around the Kingdom of Sardinia. Several states voluntarily united with Sardinia to create the Kingdom of Italy. Others, the Kingdom of the Two Sicilies and the Papal States, were conquered and annexed.
The unification of Germany was ultimately a confederal union, but it began in earnest by Prussia's annexation of numerous petty states in 1866.
Historical unions 
- Bulgarian unification in 1885, after the 1396 Ottoman conquest.
- Ukrainian unification in 1919
- Chinese reunification (1928) or "Northeast Flag Replacement" proclaimed victory of Guangzhou/Nanjing government over Beiyang government after the 1912 division.
- German reunification in 1990, divided since the 1949 division decided at the Potsdam Conference in August 1945.
- Italian unification 1815–71, divided since the 6th century Ostrogoths.
- Polish reunification in 1918–22, divided since 24 October 1795.
- Vietnam at the end of the Vietnam War in 1976, divided since 1954.
- Yemenite reunification in 1990, divided since the collapse of the Ottoman Empire in 1918.
- Denmark and the northern part of Southern Jutland in 1920. See Schleswig Plebiscites.
Unification movements 
At various times, various nationalist and irredentist movements promoted ideas of restoration or unification in various places.
Supranational and continental unions 
In addition to regional movements, supranational organizations that promote progressive integration between its members gained force in the end of the 20th century. Most of these organization are inspired on the European Union and, although most of the states are reluctant with it, the concept of unionism is often present.
Current supranational unions:
Proposed supranational unions:
- Arab Union, from Arab League
- Central Asian Union
- Community of Latin American and Caribbean States, from Rio Group and CARICOM
- North American Union, from North American Free Trade Agreement (NAFTA)
- Pacific Union, from Pacific Islands Forum
Organizations that plan to evolve into supranational unions:
- Association of Southeast Asian Nations (ASEAN)
- Caribbean Community (CARICOM)
- Central American Integration System (SICA)
- Cooperation Council for the Arab States of the Gulf (CCASG)
- Eurasian Economic Community (EurAsEC)
- Organisation of Eastern Caribbean States (OECS)
- South Asian Association for Regional Cooperation (SAARC)
- Union State (Russia and Belarus)
- Eurasian Union (Russia,Kazakhstan,Kyrgyzstan and Belarus)
Academic analysis 
Lord Durham was widely regarded as one of the most important thinkers in the history of the British Empire's constitutional evolution. He articulated clearly the difference between a full legislative union and a federation. In his 1839 Report, in discussing the proposed union of Upper and Lower Canada, he says:
Two kinds of union have been proposed – federal and legislative. By the first, the separate legislature of each province would be preserved in its present form and retain almost all its present attributes of internal legislation, the federal legislature exercising no power save in those matters which may have been expressly ceded to it by the constituent provinces. A legislative union would imply a complete incorporation of the provinces included in it under one legislature, exercising universal and sole legislative authority over all of them in exactly the same manner as the Parliament legislates alone for the whole of the British Isles.
- This is often seen as a federal union but is closer to an incorporating union as the four colonies were dissolved, their territories becoming provinces in a unitary state without any recognition as fixed constitutional entities
- ". . . that no Alteration be made in Laws which concern private Right, except for evident Utility of the Subjects within Scotland" — Article XVIII of the Treaty of Union
- Encyclopædia Britannica: "The United Kingdom of Great Britain and Northern Ireland is the political union of England, Scotland, Wales, and Northern Ireland."
- A Disunited Kingdom? - England, Ireland, Scotland and Wales, 1800-1949, Christine Kinealy, University of Central Lancashire, Cambridge University Press, 1999, ISBN 978-0-521-59844-6: "... explaining how the United Kingdom has evolved, the author explores a number of key themes including: the steps to political union, ..."
- Marianopolis College: