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A unitary state is a state or sovereign state governed as a single entity in which the central government is the supreme authority. The central government may create or abolish administrative divisions (sub-national units). Such units exercise only the powers that the central government chooses to delegate. Although political power may be delegated through devolution to regional or local governments by statute, the central government may override the decisions of devolved governments, curtail their powers, or expand their powers. The modern unitary state concept originated in France; in the aftermath of the Hundred Years' War, national feelings that emerged from the war unified France. The war accelerated the process of transforming France from a feudal monarchy to a unitary state. The French then later spread unitary states by conquests, throughout Europe during and after the Napoleonic Wars, and to the world through the vast French colonial empire.

Unitary states stand in contrast to federations, also known as federal states. A large majority of the UN member countries, 166 out of 193, have a unitary system of government, while significant population and land mass is under some kind of federation.