RC Majumdar makes no bones about the root cause behind the downfall of the mighty Mauryan Empire. The cause of course being the overt renunciation of military might by Ashoka. Let the master himself speak:

When the strong arm of Askoka, who possessed the power - to punish in spite of his repentance, was withdrawn, the Greeks poured once more into the Kabul Valley. the Punjab and even the Gangetic region and threw all these provinces into confusion.

Asoka eschewed military conquest after the Kalinga war … and called upon his descendants not to entertain any thought of aggressive warfare… unlike the earlier kings of the dynasty who often took the field in person, the last of the Mauryas lost touch with his armed forces and ceased to command their affection.

…when his [Ashoka’s] strong arm was withdrawn, central control apparently became slack. Some of the outlying provinces seceded from the empire, and the process of disintegration was accelerated…

Herein lies an important lesson. A state that eschews the power to punish, will perish.