According to the historians, the sole cause behind the decline of the Mauryas after Ashoka was his weak successors. After Ashoka's demise, there was none among his heirs to equal the gigantic task of maintaining unity within the vast Empire. Moreover the successors of Ashoka had been reared in the tradition of non-violence and the policy of Dharma Vijaya. To them, aggressive imperialism initiated by Chandragupta Maurya was a dim idea. As a result they had neither will nor the strength to bridle the process of disruption within the Empire. None of Ashoka's successors except Dasharatha could really understand and implement the Dharma Vijaya policy inaugurated by the enigmatic man. The later Mauryas followed the policy of Dharma Vijaya only by forbidding any armed resistance against the invaders and internal revolutionaries. As a result the very foundation of the Mauryas was shattered.
Apart from their weakness, another primary cause for the dismemberment of the Mauryan Empire was the ambition of the Maurya princes. The princes ruling in the provinces identified themselves with the spirit of local autonomy. They denied their allegiance to the central authority and raised the standard of revolt challenging the sovereign authority. Moreover the Rajukas were entrusted with the autonomous power in the province, because after the death of Ashoka, the weak successors could not control the revolts raised by the powerful Rajukas. Moreover since the Mauryan princes were too ambitious, they demanded for the partition of the Mauryan Empire. As a result, the vast Maurya Empire during its dying days was disintegrated into several parts. The governors or the independent head of the provinces, later allied with the Yavanas to challenge the central authority at Pataliputra. Thus Mauryan Empire began showing signs of breaking up after the death of Ashoka.
Thirdly, the Mauryan Government became unpopular because of the high handedness of the provincial governors. The provincial governors particularly committed oppression and the people revolted. The indication of ministerial revolt is also found during the reign of Bindusara; the Kalinga Rock edicts testify the ministerial revolt in his reign. Though the previous rulers were competent enough to subdue the revolt, the weak successors of Ashoka could not hold it back. The theory of ministerial revolt is found in the Puranas and in contemporary literary records. The theory of the Puranas is also corroborated in the later ages by Romila Thapar.
During the Later Mauryan period, the Maurya court and the nobilities were divided into two rival sections. One of these was headed by Pushyamitra Shunga and the other by a minister, who somehow managed to appoint his own sons as the governors of Vidarbha and Vidisha. The clash between the rival groups in the Mauryan court destroyed the vigour of administration.
The downfall of the Mauryan Empire was inevitable under the incapable Mauryas in the later ages. But Dr. Koshambhi had pointed out that economic decline is the sole cause behind the downfall of the Mauryas. The Mauryas suffered from the exhaustion of the royal exchequer owing to the enormous charitable offerings made by Ashoka and his successors to the Buddhist Bhikshus and the Sramanas. Later the royal exchequer was so exhausted that the Mauryan kings enhanced taxes and imposed tax revenue even on actors and prostitutes. The Mauryan punch-marked coins proved the sign of debasement. But Romila Thapar holds that material prosperity of the Mauryas do not present the picture of declining economy of the Mauryan dynasty. However most of the historians hold that the sole cause of the Mauryan downfall was exhaustion of economy and the weak successors could not restructure the shattered economy in time.
Apart from the above primary causes, historians have pointed out two other causes for the decline of the Mauryan supremacy in ancient India. According to them, the policy of non-violence introduced by Ashoka and his Buddhist policy were no less important as a cause for the downfall of the Mauryan Empire.
A group of historians headed by Shastri had defined that Ashoka's Buddhist policies and the prohibition of animal sacrifices and his introduction of Dana Samata and Vyavahara Samata provoked the Brahmanical community. Antagonised with the contemporary policy of promoting only one religion-Buddhism, they organised a revolt against the last surviving king Brihadratha and assassinated him. However the theory put forward by Shastri had been refuted by Dr. H.C. Roychowdhury on the ground that there are no authentic reports supporting the views of the Shastri. He suggested that Ashoka was not a fanatic and was tolerant to other religions also. His successor Jalauka was praised by the Brahmana historian Kalhana. According to Roychowdhury, there was nothing called the Brahmanical revolt, rather Pushyamitra was merely coup d' etat and killed an effete ruler, who had almost lost his authority.
The policy of non-violence, according to some historians is a powerful cause for the downfall of the Mauryan Empire. Ashoka's advocacy of non-violence and the principles f Dharma Vijaya demoralised the army as well as bureaucracy in the successive ages. However scholars have refuted the views of Roychowdhury on several grounds. According to them, Ashoka though had advocated the policy of non-violence, he did not let lose the grip of administrative machinery. Rather during his time Mauryan administration was very methodical and well maintained. Therefore the historians have opined that if his successors followed the policy of non-violence initiated by Ashoka, they should have also followed the technique of managing the administrative machinery properly from him. But the successors followed the policy of non-violence but could not maintain the Empire appropriately. Hence the policy of non-violence cannot be held responsible for the weak maintenance of administration during the later Mauryas. Hence there is a keen controversy among the scholars regarding the fact that whether the policy of non-violence can be at all held responsible for the collapse of the Mauryan Empire.
Later scholars like Romila Thapar have surmised several basic causes for the downfall of the Mauryan Empire. According to her since the Mauryan Empire was highly centralised, a direct control and responsibility was highly demanding. After Ashoka, his successors could not maintain that control, which resulted into complete disintegration of the Empire. Moreover the officials were recruited from the privileged group of men, who formed a community of their own detached from the common people as well as from the king himself. This resulted in partisan politics, which threatened the very foundation of the Mauryan dynasty. The centralised Mauryan government lacked the balanced and extensive public contact. Furthermore, when the internal conflicts accelerated the process of disintegration, the Yavana invasion under Euthydemus and Demetrius sped up the downfall of the Mauryan Empire. The impact of the Yavana invasion shattered the control of the Mauryas in the provinces particularly in the northwest. Finally, when the external and the internal forces were accelerating the breakdown of the Mauryan government, the coup d'etat by Pushyamitra Shunga in 185 B.C. , ultimately culminated in total ruination of the Mauryan Empire.