Indus Valley civilisation is one of the oldest civilisations developed indigenously in the area of Sindhu river. Sumerian/Mesopotaemian civilisation Egyptian civilisations were contemporary since there's evidence of trading between these regions.
The reason for it's decline may be due to famines epidemics and may be the bronze age collapse in egypt and mesopotaemia may have affected the trade too.
Nomadic pastoralists from steppes entered the region much later and by then the IVC people seemed to have moved to south and to the east
There may not have been an invasion but conflicts between people who arrived later in the region and the people who are already living there must have definitely happened before they settled down to live together in the region.
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