A federation (also known as a federal state) is a political entity characterized by a union of partially self-governing provinces, states, or other regions under a central federal government (federalism). In a federation, the self-governing status of the component states, as well as the division of power between them and the central government, is typically constitutionally entrenched and may not be altered by a unilateral decision of either party, the states or the federal political body. Alternatively, federation is a form of government in which sovereign power is formally divided between a central authority and a number of constituent regions so that each region retains some degree of control over its internal affairs. It is often argued that federal states where the central government has the constitutional authority to suspend a constituent state's government by invoking gross mismanagement or civil unrest, or to adopt national legislation that overrides or infringe on the constituent states' powers by invoking the central government's constitutional authority to ensure "peace and good government" or to implement obligations contracted under an international treaty, are not truly federal states.
The governmental or constitutional structure found in a federation is considered to be federalist, or to be an example of federalism. It can be considered the opposite of another system, the unitary state. France, for example, has been unitary for multiple centuries. Austria and its Bundesländer was a unitary state with administrative divisions that became federated through the implementation of the Austrian Constitution following the 1918 collapse of Austria-Hungary. Germany, with its 16 states, or Bundesländer, is an example of a federation. Federations are often multiethnic and cover a large area of territory (such as Russia, the United States, Canada, India, or Brazil), but neither is necessarily the case.
Several ancient chiefdoms and kingdoms, such as the 4th-century BCE League of Corinth, Noricum in Central Europe, and the Haudenosaunee Confederation in pre-Columbian North America, could be described as federations or confederations. The Old Swiss Confederacy was an early example of formal non-unitary statehood.
Several colonies and dominions in the New World consisted of autonomous provinces, transformed to federal states upon independence (see Spanish American wars of independence). The oldest continuous federation, and a role model for many subsequent federations, is the United States. Some of the New World federations failed; the Federal Republic of Central America broke up into independent states less than 20 years after its founding. Others, such as Argentina and Mexico, have shifted between federal, confederal, and unitary systems, before settling into federalism. Brazil became a federation only after the fall of the monarchy, and Venezuela became a federation after the Federal War. Australia and Canada are also federations.
Germany is another nation-state that has switched between confederal, federal and unitary rules, since the German Confederation was founded in 1815. The North German Confederation, the succeeding German Empire and the Weimar Republic were federations.
Founded in 1922, the Soviet Union was formally a federation of Soviet republics, autonomous republics and other federal subjects, though in practice highly centralized under the government of the Soviet Union. The Russian Federation has inherited a similar system.
Nigeria, Pakistan, India and Malaysia (then Federation of Malaya) became federations on or shortly before becoming independent from the British Empire.
In some recent cases, federations have been instituted as a measure to handle ethnic conflict within a state, such as Bosnia and Herzegovina, and Iraq since 2005.
With the United States Constitution having become effective on 4 March 1789, the United States is the oldest surviving federation. On the other end of the timeline is Nepal, which became the newest federation after its constitution went into effect on 20 September 2015.