There is a village named Assur in Persian region (now in Iraq), wherein Mediterranean (Med/Megh-as called in Saptasindhu region) used to worship their ‘God’ named ‘Assur’, as per mythical information pertaining to the period around 2100 BC. The written hostory for this story is not available, because the recording of written historical events had not developed during this primitive period. Followers of faith ASSUR, with process of a long period of time had developed their faith into a formless monotheistic God.
Monotheistic religionists were not liked by those who believed in existence of multiplicity of Gods/Goddesses. Those who opposed Monotheistic faith mostly belonged to Aryans from Central Asia who had settled in Iran; because the professional priesthood was to gain from multiplicity of Gods only.
This lead to clash of interests among both Monotheistic & the Aryan tribes.
The period between 3000 BC & 1000 BC frequently used to witness movement of various tribes in various religions, as per their needs depending upon cyclical climatic changes, periodic, famines, spread of diseases, etc., in addition to clashes among various groups of tribes.