When Asoka Maurya breathed his last, there was no able successor of that great emperor to keep the vast Maurya Empire united and strong.
It was a tragedy with the ancient system of monarchy that the entire structure of administration centered around the personality of the monarch.
As long as the king was a capable ruler, the government worked successfully. But, when the person of the king showed signs of weakness, the system of governance also decayed into weakness. This is exactly what happened to the Maurya monarchical establishment when Asoka died, leaving behind him no worthy successor to govern the first all-India great empire.
Owing to this weak succession, the remote areas of the Asokan empire soon became independent. Elsewhere, the foundation of the administration began to crumble. Asoka’s successors were so unworthy that within less than fifty years of the death of that mighty monarch, the Maurya empire disintegrated to a vanishing point. It is gathered from some Puranic descriptions as well as from the famous work ‘Harsha Charita’, written by Bana several centuries later, that the last King of the Maurya Dynasty named Brihadratha was killed by his own General named Pushyamitra Sunga in 185 or 186 B.C.
With that incident, Pushyamitra captured the throne of Magadha and founded a new dynasty known as the Sunga Dynasty. By that time, the empire of Magadha was not as vast as it was before. Distant regions of the empire were already independent. The territory of Kalinga which Asoka conquered with great difficulty was no longer under Magadha. So was case with some other territories of the earstwhile Magadhan empire. As far as it is known, when Pushyamitra came to the throne of Pataliputra, the Magadhan territory extended from Jalandhar in the north to the river Narmada in the south.
Much is not known about the origin of the Sungas. It is understood from some sources like Panini’s writings that the Sungas were Brahmins by caste. In those days, Brahmins were entitled to undergo military training like the Kshatriyas and enter into the royal armies. That being so, the Brahmin Pushyamitra could join the Maurya army, and by virtue of heroic deeds, he could become the supreme commander of the royal forces. It is mentioned in Bana Bhatta’s Harsha-Charita’ that one day General Pushyamitra invited the worthless Maurya Emperor Brihadratha to observe an army parade, and killed him there.
It is rather a rare occurrence in ancient history that a Commander-in-Chief could murder a ruling king in the presence of the, royal army. It proves that Pushyamitra was able to keep his soldiers united under his personal control and engineered a military coup to overthrow a legitimate king. In some of the Puranas also Pushyamitra was described as a General who killed the king to come to the throne.