A political union is a type of state which is composed of or created out of smaller states. The process is called unification. Unifications of states that used to be together and are reuniting is referred to as reunification. 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:
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.
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 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.
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.
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. Formally the union in each territory was sanctioned by a popular referendum, formally asking the people to agree to have as new ruler Vittorio Emanuele II (the King of Sardinia) and his legittimate heirs.
The unification of Germany was ultimately a confederal union, but it began in earnest by Prussia's annexation of numerous petty states in 1866.
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.
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
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, ..."