Final answer:
The Harappan civilization of the Indus River Valley declined due to invasion and drastic environmental change.
Step-by-step explanation:
The Harappan civilization of the Indus River Valley declined due to invasion from central Asia & Iran and drastic environmental change.
The invasion from central Asia & Iran, also known as the Aryan invasion, brought about significant changes in the region and disrupted the existing social and political structures.
Additionally, drastic environmental changes, including shifts in the patterns of rainfall and river courses, likely contributed to the decline of the civilization.
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