Pusyamitra Sunga was originally a Senapati (General) of the Mauryan empire. In 185 BCE he assassinated the last Mauryan Emperor (Brhadrata) during an army review, and proclaimed himself King. He then performed the Ashwamedha Yajna (horse sacrifice) and brought much of Northern India under his rule. Inscriptions of the Shungas have been found as far as the Jalandhar in the Punjab, and the Divyavadana mentions that his rule extended as far as Sagala (Sialkot).
Pushyamitra's reign was marked by warfare which was characteristic of this age in India. He and his successors fought the Indo Greeks, Kalingas, Satavahanas (Andhras), and possibly the kingdoms of Panchala and Mathura (which may not have been under his rule).
Following the assassination of Brhadrata, in 180 BCE the Greco-Bactrian Kingdom invaded Sunga lands with an army led by Demetrius I of Bactria. The Greco-Bactrians overran Arachosia, Gedrosia, the Punjab, and perhaps Mathura. Pushyamitra may have regained some of the lost territory, with the Ashvamedha terminating at the Indus late in his reign (Kulke? ).
Pusyamitra may have been attacked by King Kharavela of Kalinga (modern Orissa). Kharavela's inscriptions claim that he forced a King of Magadha named "Bahasatimita", (thought to be the Sunga King Brhaspatimitra, or Pusyamitra himself) to bow at his feet. However, this has not been confirmed as dates for Kharavela range several centuries.
According to the 2nd century Ashokavadana:
A Buddhist tradition holds him as having taken steps to check the spread of Buddhism as "the number one enemy of the sons of the Shakya's and a most cruel persecutor of the religion". The Divyavadana ascribes to him the razing of stupas and viharas built by Ashoka, and describes him as one who wanted to undo the work of Ashoka.
Among the detractors is Romila Thapar, who writes that archaeological evidence casts doubt on the claims of Buddhist persecution by Pushyamitra. Support of the Buddhist faith by the Sungas at some point is suggested by an epigraph on the gateway of Barhut, which mentions its erection "during the supremacy of the Sungas".
On the other hand, Sir John Marshall noted that the Sanchi stupa was vandalized during the 2nd century before it was rebuilt later on a larger scale, suggesting the possibility that the original brick stupa built by Ashoka was destroyed by Pusyamitra and then restored by his successor Agnimitra. Similarly, the Deokothar Stupas (geographically located between Sanchi and Barhut) suffered destruction during the same period, also suggesting some kind of involvement of Sunga rule. Proponents also point to the proclamations and claim that the Manu Smriti was propagated.