What type of government system allows one person to wield ultimate authority over a country and its people?

The type of system of government allows a person to exercise maximum authority over a country and its people is the dictatorial system

the dictatorial system can be defined as an authoritarian political regime maintained by violence of an exceptional and illegitimate nature. It can be led by a person or a group that imposes its project of government on society with the help of force.

These governance systems tend to be fairly centralized. Power is extremely concentrated in the hands of the person or group that governs the state, with little or no openness to political debate. The spaces of communication and deliberation are usually tightly regulated or suppressed. This includes the press, the legislature and judiciary, and political parties, which are often forbidden to exist.

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in 550 b.c.e. cyrus the great, the leader of the persians, conquered the medes and united the iranian people under one ruler for the first time. cyrus became the first king of the persian empire and went on to establish one of the largest empires in world.