Political union

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 or real 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:

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").

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 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, more frequently, by conquest.

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.

(Arguably Hawaii with the United States of America is an example, but Hawaii was first annexed without statehood in 1898.)

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 legitimate heirs.

...
Wikipedia