Mauryas maintained a huge army, a vast bureaucracy and ruled over a large part of the Indian subcontinent. But soon after the death of Ashoka the empire got divided into two parts.
While king Dasaratha controlled the eastern part of the empire, the western part was under Samprati.
Why did such a large empire decline so early? Some historians believe that Ashoka under the influence of Buddhism became a pacifist and weakened his army. It is also said that the religious policy of Ashoka antagonised the brahmanas as he banned the animal sacrifice, which affected the economic and religious activities of the brahmanas. Therefore, Pusyamitra, the brahmana chief of the army, killed the last Mauryan king. But this does not seem to be correct as the study of Ashokan inscriptions reveals that Ashoka paid full respect to brahmanas.
Moreover it is true that Ashoka followed a policy of peace and harmony, but he did not disband his army and was always prepared to face any eventuality.
One of the main reasons for decline could be the succession of weak rulers. Who could not keep under check those, ministers and officials of far-flung regions, who had become oppressive and acted against the interest of the centre. It is also possible that Mauryan rule may have suffered some kind of economic crisis.
It is reflected in the debasement of some coins of that period. This crisis might have developed either due to massive donations and charity or overspending on the imperial administrative system. In fact, the reason of decline was inherent in the structure of the vast centralized empire itself.
The successors of Ashoka could not maintain the balance between the centre and the various provincial governors of the empire, and at the first possible opportunity, they made an effort to separate themselves from the centre. However, Mauryan empire though declined had a positive effect of spreading agriculture and iron technology in the different parts of the subcontinent. It facilitated the rise of several regional kingdoms in the post-Mauryan period.