A political union is a type of political entity which is composed of, or created from, smaller polities or the process which achieves this. These smaller polities are usually called federated states and federal territories in a federal government; they are called prefectures, regions, or provinces in the case of a centralised government. This form of government may be created through voluntary and mutual cession and is described as unionism by its constituent members and proponents. In other cases, it may arise from political unification, characterised by coercion and conquest. The unification of separate states which, in the past, had together constituted a single entity is known as reunification. Unlike a personal union or real union, the individual constituent entities may have devolution of powers but are subordinate to a central government or coordinated in some sort of organization. In a federalised system, the constituent entities usually have internal autonomy, for example in the setup of police departments, and share power with the federal government, for whom external sovereignty, military forces, and foreign affairs are usually reserved. The union is recognised 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 many forms, broadly categorized as:

In an incorporating union a new state is created, the former states being entirely dissolved into the new state (although some aspects may be preserved; see below).

Incorporating unions have been present throughout much of history, such as when:

Nevertheless, a full incorporating union may preserve the laws and institutions of the former states, as happened in the creation 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. These guarantees may be to ensure the success of a proposed union (or in the least to prevent continuing resistance), as occurred in the union of Brittany and France in 1532 (Union of Brittany and France) in which a guarantee was given for the continuance of laws and of the Estates of Brittany (a guarantee revoked in 1789 at the French Revolution). The assurance that institutions are preserved in a union of states can also occur as states realize that, whilst a power imbalance exists (such as between the economic conditions of Scotland and England prior to the Acts of Union 1707), it is not so great that it precludes the possibility for concessions to be made. The Treaty of Union for creating the unified 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), which was significant for both parties. The Scottish, despite economic troubles during the Seven Ill Years preceding the union, still retained power to negotiate.

This marks a delineation of states that are able to ensure preservation of interests: there has to be some mutually beneficial reasoning behind the formal or informal preservation of interests. 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 the Kingdom of Ireland, though they were continued as a matter of practice. The informal recognition of such interests represents the different circumstances of the two Unions, the small base of institutional power in Ireland at the time (those who were the beneficiaries of the Protestant Ascendancy) had faced a revolution in the Irish Rebellion of 1798, and as a result there was an institutional drive toward unification, limiting the Irish negotiating power. However, informal guarantees were given to preclude the possibility of further Irish unrest in the period following the French Revolution of 1789 and the 1798 rebellion. These types of informal arrangements are more susceptible to changes; for example, Tyrol was guaranteed that its Freischütz companies would not be posted to fight outside Tyrol without their consent, a guarantee later revoked by the Austrian state. However, this case can be contrasted with the continued existence of the Scottish Parliament and a separate body of Scottish law distinct from English law.

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.

Incorporating annexations have occurred at various points in history, such as when: