The decline of the Indus-Civilization came also with the invasion of the Aryans.
They were tall and white coloured men.
The Aryans were people who had first migrated from the Urals to the Central Asian Steppes and thence to the Indian Subcontinent. They were a warlike primarily pastoral people who pursued a transitory lifestyle moving from pasture to pasture for their herds of cattle and horses.

The Aryans left there former land after it became infertile probably due climate changes. One group settled in Northern-Greece and an other group in Iran.
One split off of the Iranian group invaded the Indian subcontinent.

The Aryans were less developed compare to the urban inhabitants in the Indus Valley but they were more skilled in weaponry and metal processing. After a while they and were beginning to appreciate the value of agriculture and a settled existence. The first kingdoms arose. The Aryan leaders became kings and the people started to form occupational groups.
When the Aryans arrived they brought their religion with them. Since they were not a settled race and pastoral their gods were mostly elemental. The early gods were mostly deified aspects of nature. Later the gods acquired complex mythologies and had been personalized or anthropomorphized.
The history of the Aryans is known since the actual writing of the Vedas. The earlier distinct roles they had been accorded had been blurred by overlapping of qualities and duties. This is why the first three parts of the Vedas – The Samhitas, The Brahmanas and The Aranyakas – are so preoccupied with ritualistic worship.