There are several theories regarding the decline of the Indus Valley Civilisation. Here are the most prominent reasons: 1. Environmental factors: Geological surveys show that the Indus Valley experienced a decrease in rainfall, which led to drought and affected agricultural productivity. 2. Decline in trade: The deterioration of trade networks, possibly due to shifts in river courses or changes in international trade patterns, resulted in economic instability. 3. Aryan invasion: Some theories suggest that the Aryan invasion from the northwest disrupted the Indus Valley Civilisation's social structure and governance. 4. Epidemics and disease: The spread of diseases may have weakened the population and hampered recovery efforts. Following the decline of the Indus Valley Civilisation, various post-Mauryan Kingdoms rose to power in the Indian subcontinent, such as the Magadha Empire, Mauryan Empire, Gupta Empire, and various regional kingdoms. These new powers reshaped the political landscape of ancient India.