Maybe it was a natural calamity that crushed the older civilization forever..it is also claimed first that Aryans were the one who are the reason behind the civilization 's decline..because they had a better equipment and knew the use of Iron.Well now it is not claimed so because later it was determined and concluded that Aryans saw these already declining cities and moved forward.

**The Decline of the Harappan or the Indus Valley Civilization**

The Harappan or the Indus Valley Civilization was one of the earliest urban civilizations that flourished in the Indus River valley, covering present-day Pakistan and northwest India, from approximately 2600 BCE to 1900 BCE. While the exact reasons for its decline are still debated among scholars, several factors likely contributed to its eventual collapse.

**1. Environmental Factors:**
- **Climate Change:** One significant environmental factor that may have contributed to the decline of the Indus Valley Civilization is climate change. The region experienced a shift in the monsoon patterns, leading to a decrease in rainfall and subsequent droughts. This would have severely impacted agricultural productivity and the availability of water for irrigation.
- **River Diversion:** The Indus River, which played a crucial role in sustaining the civilization, changed its course over time. This natural phenomenon would have disrupted the agricultural systems and trade networks that relied heavily on the river for transportation and irrigation.

**2. Economic Factors:**
- **Trade Disruption:** The decline of long-distance trade networks could have had a detrimental effect on the economy of the Indus Valley Civilization. The weakening of trade connections with Mesopotamia and other regions might have led to a decline in the availability of valuable resources and raw materials.
- **Resource Depletion:** The intensive agricultural practices and deforestation might have led to soil erosion, depletion of natural resources, and declining agricultural productivity. This could have resulted in food shortages and economic instability.

**3. Social and Political Factors:**
- **Internal Conflicts:** The Indus Valley Civilization was composed of several city-states, and internal conflicts for control over resources and power may have weakened the social and political fabric of the civilization.
- **Invasion and External Threats:** The arrival of the Indo-Aryans from the northwest has been suggested as a possible factor contributing to the decline. The migration and subsequent invasion by these nomadic groups could have disrupted the established social order and led to the downfall of the civilization.

**4. Cultural Factors:**
- **Religious and Cultural Changes:** Changes in belief systems, religious practices, or cultural values might have impacted the social cohesion and stability of the civilization. However, the extent to which these factors contributed to the decline remains speculative.

In conclusion, the decline of the Harappan or the Indus Valley Civilization was likely influenced by a combination of environmental, economic, social, political, and cultural factors. The interplay of these factors, such as climate change, trade disruption, internal conflicts, external invasions, and cultural changes, likely led to the eventual collapse of this once-thriving urban civilization.