Monarchy and the Divine Right to rule 
In most of the oldest river valley civilizations, the strongest military man became king.  A political system in which government is controlled by a king or queen is called a monarchy.  Some kings were also the chief priest in their city.  This dual role was not uncommon, and served to place the king on a much higher social level than his subjects, thus making it easier for him to govern them

Some kings took the religious affiliation even further, declaring they had the right to rule because God had chosen them to do so.  Claiming to rule by divine right made kings appear very powerful, and closely connected to the patron god of the city.  

Examples of Early Monarchy
Mesopotamia
In Sumer and Babylon, the king was often the monarch and the high priest.  The position of these so-called priest-kings was relatively unstable.  If the god(s) didn't provide for welfare of the citizens, the people often held the king responsible, and deposed him.

Egypt 
In the land of the Nile, the all-powerful monarch was called pharaoh.  The people thought pharaoh was the physical incarnation of the god Horus here on earth.  As both a man and a god, pharaoh blended and used both politics and religion to govern his lands.