One explanation is that it was due to foreign invasion. Advocates of this theory believe that the Indus civilization was destroyed by the Indo-European-speaking Aryans, as evidenced by the great god "the destroyer of the citadel of Indraprastha", as described in the largest layer of fights and violent killings at the city ruins. However, the earliest Aryan entry into the region was in the 15th century B.C., by which time the civilization had already declined. Also, the decline of the Indus civilization was a degradation of the urban economy to a village economy and nomadic pastoralism, not a depopulation caused by large-scale warfare.

Some argue that it was caused by floods, that the Indus was diverted by massive floods, and that people had to abandon their old cities. However, this theory does not explain the decline of large cities throughout the Indus Valley.

Other theories suggest that the Indus Valley was cultivated by floods, which transmitted diseases such as malaria, and that the widespread spread of disease was one of the causes of the decline of the Harappan civilization. In addition, from the ecological and hydrological aspects, Harappan urban civilization in the late stage of the drastic geological and hydrological changes, such as some areas due to the gradual drying up of the river, the disappearance of settlements, affecting the civilization of the development and maintenance of the civilization, and contribute to the rapid decline of the civilization.

Newly it has been pointed out that the cause of the decline of the Harappan civilization should be focused on why it did not have the power to resist natural and man-made disasters, Harappan civilization is engaged in large-scale internal and external trade and handicrafts, but the basis of the limited level of production of agriculture and pastoralism. This was the result of the stagnation of the system, which was characterized by the use of rudimentary tools to cultivate the floodplains of the rivers, the absence of decent irrigation works, the slowness of production techniques and the lack of iron tools. Under such circumstances, the impact of foreign culture or natural disasters could easily deal a fatal blow to the old civilization.