The Mauryan Empire was one of the largest, albeit short-lived, empires to ever rule over the Indian subcontinent.
It is impossible to discuss the decline of this empire without talking about a particular man. That man is Ashoka, better known as Ashoka the great. He ascended the throne in 268 BC. Under him the empire reached it’s greatest extent. This man would succeed at everything he would put his mind to. Eventually he would wage a destructive and bloody war against the Kalinga state. The state of Kalinga was very different from the Maurya empire. Much like Athens, Kalinga prided itself on democracy, an immature democracy, but a democracy nonetheless.
The Kalinga war would end in 260 BC with it’s conquest. This war resulted in over 100,000 dead and deportation of many more. Ashoka, after his hard earned victory, would walk on his newly conquered territory and would be horrified. He saw the horrors of the war that he had started. Entire villages burnt to the ground and corpses covering the very ground that he walked upon. This affected Ashoka deeply. He renounced violence and would accept Buddhism as his religion. Afterwards Buddhist monks would be sent to every corner of the world by him to spread the way of peace. In 232 BC Ashoka died, leaving behind an empire at it’s greatest.
The decline of the Maurya Empire began with Ashoka’s death. As seen in history many times before, a strong king being succeeded by weak kings usually spells disaster. For the Maurya this was even worse. A line of weak kings would follow Ashoka, each one weaker than the one before. This would go on for 50 years before the Empire finally fell.
Another reason for the decline was inevitability. The empire had grown too large and had become impossible to administer. Since all power was centralized in the capital and ruled by a king, the outlying regions could no longer be administered. The different regions would slowly rebel and declare independence and the incompetent kings would be unable to return these regions back into their control.
The invasions from the Greco-Bactrians from the west would further weaken the empire. The weak leaders would attract invasions from the west. The Maurya would no longer be able to hold of the invasions.
The killing blow to the empire would come from within. In 185 BC the last Maurya king, Brihadratha, was assassinated during a military parade by a military commander, Pushyamitra Shunga. Pushyamitra would take the throne for himself and form the Shunga Dynasty.
Summarizing, I would say that the decline of the Mauryan Empire began in 232 BC, just as Ashoka died. Ashoka would be followed by a series of weak and incompetent kings each further contributing to the decline of the empire, till 185 BC, when a general would kill the last of the line and put the Empire out of it’s misery.