The Mauryan territories, centred on the capital of Pataliputra, had shrunk considerably from the time of Ashoka to when Brihadratha came to the throne.
According to the Puranas, Brihadratha succeeded his father Shatadhanvan to the throne and ruled for three years.
In 185 BCE just after the assassination of Brihadratha by his army chief Pushyamitra, Greco-Bactrian king Demetrius (Dharmamita) invaded northwestern India (parts of modern-day Afghanistan and Pakistan) and occupied it.
A key detail is mentioned by Sri Lankan Buddhist monks in the Paramparapustaka chronicle, pointing that Brihadratha married Demetrius' daughter, Berenice (Suvarnnaksi in Pali language).
[4] The Greco-Bactrians may also have invaded India in order to protect Greek populations in the subcontinent.